Domain names aren't case sensitive, but the directory structure that follow it is depending on your server make, I believe.
Apache is case sensitive; I don't think ISS is (I could always be wrong, there--it could be a Windows/Unix thang :-). In Apache, you can't use mod_rewrite to make files case insensitive, which would have been my first thought. But they seem to suggest that you can with mod_speling (sic, ironically ;-). 'Never tried it myself though, and of course, your server has to have it. :-)