Some good links to commercial solutions for Geo/IP detection and redirection and a great php example of how to redirect based on the HTTP_ACCEPT_LANGUAGE header sent with browsers.

I've never looked into it very closely but there are problems with both methods I think:

IP redirection can be quite inaccurate as I understand it and people like me who live abroad but do not speak the native language well hate being redirected..

HTTP_ACCEPT_LANGUAGE: That's great, but what if im again, living abroad? I could be browsing in english but require a (in my case) danish version of the site to actually buy whatever they are selling. Neither seems like a complete solution on it's own, you'd still have to have some pretty prominent links for mistakes and unusual circumstances wouldnt you?

Probably be easier not to bother and just have a cookie set for LANG and if it doesnt exist, show the user a language choice page no?