According to wikipedia un*x is correct, but according to my experience (and wikipedia) *nix is about a million times more common.
If you're going to say un*x you might as well wear ripped jeans, drop out of school*, quit taking showers and get used to using Starbucks wifi, since that's the only place you'll get the Internet once they kick you out of the college library.
*Alternatively switch to an art major.