iOS and unarchiving xib files

I'm finishing off an app for iOS 3.0 and up to run on the iPhone and iPod Touch. It's working fine on iOS4.0 and up, but when I backtest it on a second generation iPod Touch running iOS 3.0 the launch image shows and I'm met with the following:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSKeyedUnarchiver initForReadingWithData:]: incomprehensible archive (0x4e, 0x49, 0x42, 0x41, 0x72, 0x63, 0x68, 0x69)'
2012-01-15 09:35:53.025 MyApp[3026:207] Stack: (
    808001701,
    805397928,
    807551015,
    807550919,
    810902709,
    815043196,
    815042704,
    814682908,
    814682012,
    814970032,
    814968048,
    814966908,
    839149932,
    807750263,
    807747947,
    814678868,
    814672532,
    61527,
    10764
)
terminate called after throwing an instance of 'NSException'
(gdb) backtrace
#0  0x31dd594c in __kill ()
#1  0x31dd5940 in kill ()
#2  0x31dd5934 in raise ()
#3  0x31deccfe in abort ()
#4  0x374ef848 in __gnu_cxx::__verbose_terminate_handler ()
#5  0x300166c0 in _objc_terminate ()
#6  0x374ecf3c in __cxxabiv1::__terminate ()
#7  0x374ecfb4 in std::terminate ()
#8  0x374ed0dc in __cxa_throw ()
#9  0x300165e4 in objc_exception_throw ()
#10 0x30224026 in +[NSException raise:format:arguments:] ()
#11 0x30223fc6 in +[NSException raise:format:] ()
#12 0x305564b4 in -[NSKeyedUnarchiver initForReadingWithData:] ()
#13 0x3094927c in -[UINib instantiateWithOptions:owner:loadingResourcesFromBundle:] ()
#14 0x30949090 in -[NSBundle(NSBundleAdditions) loadNibNamed:owner:options:] ()
#15 0x308f131c in -[UIApplication _loadMainNibFile] ()
#16 0x308f0f9c in -[UIApplication _runWithURL:sourceBundleID:] ()
#17 0x309374b0 in -[UIApplication handleEvent:withNewEvent:] ()
#18 0x30936cf0 in -[UIApplication sendEvent:] ()
#19 0x3093687c in _UIApplicationHandleEvent ()
#20 0x3204696c in PurpleEventCallback ()
#21 0x30254a76 in CFRunLoopRunSpecific ()
#22 0x3025416a in CFRunLoopRunInMode ()
#23 0x308f0354 in -[UIApplication _run] ()
#24 0x308eea94 in UIApplicationMain ()
#25 0x0000f056 in main (argc=1, argv=0x2ffff7c8) at ...... Classes/main.m:16
Current language:  auto; currently objective-c

Any ideas what this is and how to fix it?

I'm building in Xcode 4.2.1.

PS - The problem is not resolved by cleaning the build or by resaving .xib files. Sometimes the app will get past this stage but will crash for apparently the same reason when it sees a later .xib, say in loading a view controller.

This answer I believe covers what Ikuragames might have given (he's not near his computer). On his behalf, to solve the problem in Xcode 4.2.1 click on the .xib file and in the file properties panel on the right hand side, set the deployment to "iOS 3.0" (in my case) and development to "Interface Builder 3.0" (again in my case). Do this for every single .xib file in the project.

It's worth keeping in mind that Xcode 4.2.1 might well set defaults of "iOS 5.0" for the deployment here, to match the current SDK, and "Xcode 4.1" for the document type. Why Xcode doesn't just match the deployment target with the project deployment target I'm not sure...

If this still doesn't work or you're very paranoid by this stage, catch the exception raised by the NSKeyedUnarchiver when loading the .xib file and there create the view programmatically.