Should I default my website to www.foo or not?

Question

Notice how the default domain for stackoverflow is http://stackoverflow.com and if you try to goto http://www.stackoverflow.com it bounces you to http://stackoverflow.com ?

What is the reason for this? Not the tech reason (as in the http code, etc) but why would the site owners want to do this?

I know it's purely aesthetic and I always have host-headers for both www and not, but is there a reason to bounce a user to a single domain, subheaded or not?

Update 1

Not having a subdomain is called a bare domain. Thanks peeps! never knew it had a term :)

Update 2

Thanks for the answers so far - please note I understand that www.domain.com can point to domain.com. This is not a question about if i should offer both or either/or, it's asking why some sites default to a baredomain instead of www subdomains, or vice-versa. Cheers.

Jeff Atwood actually HAS explained why he's gone for bare domains here and here. (Nod to Jonas Pegerfalk for the post :) )

Jeff's post (and others in this thread) also talk about the problems of a bare domain with cookies and static images. Basically, if you have cookies on in a bare domain, then all subdomains are forced also. The solution is to purchase another domain, as posted by the Yahoo Perf Team here.

Answer 1

I think one reason is to help with search rankings so that for each page only one page is getting rankings instead of being split between two domains.

Answer 2

It is worth noting that you can't have CNAME and an NS record on the same (bare domain) name in DNS. So, if you use a CDN and need to set up a CNAME record for your web server, you can't do it if you are using a bare domain. You must use "www" or some other prefix.

Having said that, I prefer the look of URLs without the "www." prefix so I use a bare domain for all my sites. (I don't need a CDN.)

When I am mentioning URLs for the general public (eg. on a business card), I feel that one has to use either the www. prefix or the http:// prefix. Otherwise, just a bare domain name doesn't tell people they can necessarily type it into their browser. So, since I consider http:// an ugly wart on a business card, I do use the www. prefix there.

What a mess.