I think, the only solution is to use a neato' layout and the pos attribut.
To do what you want, I start by creating a small Ruby script to calculate all nodes positions :
radius = 20
(1..128).each do |i|
x = Math.cos(((Math::PI*2)/128.0)*i.to_f)*radius
y = Math.sin(((Math::PI*2)/128.0)*i.to_f)*radius
puts " #{i}[label=\"#{i}\", pos=\"#{x},#{y}!\", shape = \"square\"];"
end
Then, I put the result in the graphviz script :
digraph G {
layout="neato"
1[label="1", pos="19.9759091241034,0.98135348654836!", shape = "square"];
2[label="2", pos="19.9036945334439,1.96034280659121!", shape = "square"];
3[label="3", pos="19.7835301992956,2.93460948910723!", shape = "square"];
4[label="4", pos="19.6157056080646,3.90180644032256!", shape = "square"];
5[label="5", pos="19.4006250638909,4.85960359806528!", shape = "square"];
6[label="6", pos="19.1388067146442,5.80569354508925!", shape = "square"];
7[label="7", pos="18.8308813036604,6.7377970678444!", shape = "square"];
8[label="8", pos="18.4775906502257,7.6536686473018!", shape = "square"];
9[label="9", pos="18.0797858624689,8.55110186860564!", shape = "square"];
10[label="10", pos="17.6384252869671,9.42793473651995!", shape = "square"];
11[label="11", pos="17.1545722000054,10.2820548838644!", shape = "square"];
12[label="12", pos="16.6293922460509,11.111404660392!", shape = "square"];
13[label="13", pos="16.0641506296129,11.9139860898487!", shape = "square"];
14[label="14", pos="15.4602090672547,12.6878656832729!", shape = "square"];
15[label="15", pos="14.8190225070992,13.4311790969404!", shape = "square"];
16[label="16", pos="14.142135623731,14.1421356237309!", shape = "square"];
17[label="17", pos="13.4311790969404,14.8190225070992!", shape = "square"];
18[label="18", pos="12.6878656832729,15.4602090672547!", shape = "square"];
19[label="19", pos="11.9139860898487,16.0641506296129!", shape = "square"];
20[label="20", pos="11.111404660392,16.6293922460509!", shape = "square"];
21[label="21", pos="10.2820548838644,17.1545722000054!", shape = "square"];
22[label="22", pos="9.42793473651996,17.6384252869671!", shape = "square"];
23[label="23", pos="8.55110186860564,18.0797858624689!", shape = "square"];
24[label="24", pos="7.6536686473018,18.4775906502257!", shape = "square"];
25[label="25", pos="6.7377970678444,18.8308813036604!", shape = "square"];
26[label="26", pos="5.80569354508925,19.1388067146442!", shape = "square"];
27[label="27", pos="4.85960359806528,19.4006250638909!", shape = "square"];
28[label="28", pos="3.90180644032257,19.6157056080646!", shape = "square"];
29[label="29", pos="2.93460948910723,19.7835301992956!", shape = "square"];
30[label="30", pos="1.96034280659122,19.9036945334439!", shape = "square"];
31[label="31", pos="0.981353486548363,19.9759091241034!", shape = "square"];
32[label="32", pos="1.22464679914735e-15,20.0!", shape = "square"];
33[label="33", pos="-0.98135348654836,19.9759091241034!", shape = "square"];
34[label="34", pos="-1.96034280659121,19.9036945334439!", shape = "square"];
35[label="35", pos="-2.93460948910723,19.7835301992956!", shape = "square"];
36[label="36", pos="-3.90180644032256,19.6157056080646!", shape = "square"];
37[label="37", pos="-4.85960359806528,19.4006250638909!", shape = "square"];
38[label="38", pos="-5.80569354508924,19.1388067146442!", shape = "square"];
39[label="39", pos="-6.7377970678444,18.8308813036604!", shape = "square"];
40[label="40", pos="-7.65366864730179,18.4775906502257!", shape = "square"];
41[label="41", pos="-8.55110186860564,18.0797858624689!", shape = "square"];
42[label="42", pos="-9.42793473651995,17.6384252869671!", shape = "square"];
43[label="43", pos="-10.2820548838644,17.1545722000054!", shape = "square"];
44[label="44", pos="-11.111404660392,16.6293922460509!", shape = "square"];
45[label="45", pos="-11.9139860898487,16.0641506296129!", shape = "square"];
46[label="46", pos="-12.6878656832729,15.4602090672547!", shape = "square"];
47[label="47", pos="-13.4311790969404,14.8190225070992!", shape = "square"];
48[label="48", pos="-14.1421356237309,14.142135623731!", shape = "square"];
49[label="49", pos="-14.8190225070992,13.4311790969404!", shape = "square"];
50[label="50", pos="-15.4602090672547,12.6878656832729!", shape = "square"];
51[label="51", pos="-16.0641506296129,11.9139860898487!", shape = "square"];
52[label="52", pos="-16.6293922460509,11.111404660392!", shape = "square"];
53[label="53", pos="-17.1545722000054,10.2820548838644!", shape = "square"];
54[label="54", pos="-17.6384252869671,9.42793473651996!", shape = "square"];
55[label="55", pos="-18.0797858624689,8.55110186860564!", shape = "square"];
56[label="56", pos="-18.4775906502257,7.6536686473018!", shape = "square"];
57[label="57", pos="-18.8308813036604,6.73779706784441!", shape = "square"];
58[label="58", pos="-19.1388067146442,5.80569354508925!", shape = "square"];
59[label="59", pos="-19.4006250638909,4.85960359806528!", shape = "square"];
60[label="60", pos="-19.6157056080646,3.90180644032257!", shape = "square"];
61[label="61", pos="-19.7835301992956,2.93460948910724!", shape = "square"];
62[label="62", pos="-19.9036945334439,1.96034280659122!", shape = "square"];
63[label="63", pos="-19.9759091241034,0.98135348654836!", shape = "square"];
64[label="64", pos="-20.0,2.44929359829471e-15!", shape = "square"];
65[label="65", pos="-19.9759091241034,-0.981353486548354!", shape = "square"];
66[label="66", pos="-19.9036945334439,-1.96034280659121!", shape = "square"];
67[label="67", pos="-19.7835301992956,-2.93460948910723!", shape = "square"];
68[label="68", pos="-19.6157056080646,-3.90180644032257!", shape = "square"];
69[label="69", pos="-19.4006250638909,-4.85960359806528!", shape = "square"];
70[label="70", pos="-19.1388067146442,-5.80569354508924!", shape = "square"];
71[label="71", pos="-18.8308813036604,-6.7377970678444!", shape = "square"];
72[label="72", pos="-18.4775906502257,-7.65366864730179!", shape = "square"];
73[label="73", pos="-18.0797858624689,-8.55110186860564!", shape = "square"];
74[label="74", pos="-17.6384252869671,-9.42793473651995!", shape = "square"];
75[label="75", pos="-17.1545722000054,-10.2820548838644!", shape = "square"];
76[label="76", pos="-16.6293922460509,-11.111404660392!", shape = "square"];
77[label="77", pos="-16.0641506296129,-11.9139860898487!", shape = "square"];
78[label="78", pos="-15.4602090672547,-12.6878656832729!", shape = "square"];
79[label="79", pos="-14.8190225070992,-13.4311790969404!", shape = "square"];
80[label="80", pos="-14.142135623731,-14.1421356237309!", shape = "square"];
81[label="81", pos="-13.4311790969404,-14.8190225070992!", shape = "square"];
82[label="82", pos="-12.6878656832729,-15.4602090672547!", shape = "square"];
83[label="83", pos="-11.9139860898487,-16.0641506296129!", shape = "square"];
84[label="84", pos="-11.111404660392,-16.6293922460509!", shape = "square"];
85[label="85", pos="-10.2820548838644,-17.1545722000054!", shape = "square"];
86[label="86", pos="-9.42793473651996,-17.6384252869671!", shape = "square"];
87[label="87", pos="-8.55110186860565,-18.0797858624689!", shape = "square"];
88[label="88", pos="-7.65366864730181,-18.4775906502257!", shape = "square"];
89[label="89", pos="-6.7377970678444,-18.8308813036604!", shape = "square"];
90[label="90", pos="-5.80569354508925,-19.1388067146442!", shape = "square"];
91[label="91", pos="-4.85960359806528,-19.4006250638909!", shape = "square"];
92[label="92", pos="-3.90180644032257,-19.6157056080646!", shape = "square"];
93[label="93", pos="-2.93460948910725,-19.7835301992956!", shape = "square"];
94[label="94", pos="-1.96034280659121,-19.9036945334439!", shape = "square"];
95[label="95", pos="-0.981353486548361,-19.9759091241034!", shape = "square"];
96[label="96", pos="-3.67394039744206e-15,-20.0!", shape = "square"];
97[label="97", pos="0.981353486548353,-19.9759091241034!", shape = "square"];
98[label="98", pos="1.9603428065912,-19.9036945334439!", shape = "square"];
99[label="99", pos="2.93460948910724,-19.7835301992956!", shape = "square"];
100[label="100", pos="3.90180644032257,-19.6157056080646!", shape = "square"];
101[label="101", pos="4.85960359806528,-19.4006250638909!", shape = "square"];
102[label="102", pos="5.80569354508924,-19.1388067146442!", shape = "square"];
103[label="103", pos="6.73779706784439,-18.8308813036604!", shape = "square"];
104[label="104", pos="7.6536686473018,-18.4775906502257!", shape = "square"];
105[label="105", pos="8.55110186860564,-18.0797858624689!", shape = "square"];
106[label="106", pos="9.42793473651995,-17.6384252869671!", shape = "square"];
107[label="107", pos="10.2820548838644,-17.1545722000054!", shape = "square"];
108[label="108", pos="11.111404660392,-16.6293922460509!", shape = "square"];
109[label="109", pos="11.9139860898487,-16.0641506296129!", shape = "square"];
110[label="110", pos="12.6878656832729,-15.4602090672547!", shape = "square"];
111[label="111", pos="13.4311790969404,-14.8190225070992!", shape = "square"];
112[label="112", pos="14.1421356237309,-14.142135623731!", shape = "square"];
113[label="113", pos="14.8190225070992,-13.4311790969404!", shape = "square"];
114[label="114", pos="15.4602090672547,-12.6878656832729!", shape = "square"];
115[label="115", pos="16.0641506296129,-11.9139860898487!", shape = "square"];
116[label="116", pos="16.6293922460509,-11.111404660392!", shape = "square"];
117[label="117", pos="17.1545722000054,-10.2820548838644!", shape = "square"];
118[label="118", pos="17.6384252869671,-9.42793473651996!", shape = "square"];
119[label="119", pos="18.0797858624689,-8.55110186860565!", shape = "square"];
120[label="120", pos="18.4775906502257,-7.65366864730181!", shape = "square"];
121[label="121", pos="18.8308813036604,-6.7377970678444!", shape = "square"];
122[label="122", pos="19.1388067146442,-5.80569354508925!", shape = "square"];
123[label="123", pos="19.4006250638909,-4.85960359806528!", shape = "square"];
124[label="124", pos="19.6157056080646,-3.90180644032257!", shape = "square"];
125[label="125", pos="19.7835301992956,-2.93460948910725!", shape = "square"];
126[label="126", pos="19.9036945334439,-1.96034280659121!", shape = "square"];
127[label="127", pos="19.9759091241034,-0.981353486548362!", shape = "square"];
128[label="128", pos="20.0,-4.89858719658941e-15!", shape = "square"];
25->42
25->71
26->25
26->40
27->30
29->25
29->26
29->27
29->30
29->32
29->40
29->80
32->39
33->28
33->44
33->74
37->34
37->66
37->69
38->60
38->107
40->100
47->30
48->35
48->36
50->35
50->63
51->50
51->96
52->50
53->51
53->96
59->50
59->51
59->52
59->60
60->50
60->63
60->95
67->74
67->114
68->74
70->74
70->126
71->74
71->86
72->70
75->39
77->81
79->73
80->84
82->78
82->114
86->115
87->115
87->121
91->69
91->87
96->30
96->114
101->107
102->108
107->75
107->78
108->95
108->103
111->80
111->114
114->128
115->114
118->128
119->103
121->72
123->116
125->80
126->122
128->96
}
Here is the result : http://dl.dropbox.com/u/72629/stackoverflow-4242949.png