2 computers - Mac OS X on the one I actually like to use, plus I keep a PC running Vista Ultimate around because I'm a gamer. Yes, I realize I could do that with another version of Vista, but student discounts offered through universities are a wonderful thing, and Ultimate was only $5 more than the "lesser" versions. Basically, I put up with its OS in order to play things like Bioshock, Counter-Strike, and (yes, I'm a nerd and I'm ok with it) Magic Online.

I was a computer science major, so I could never give up having a unix command line (windows does not count), but IMO linux is too much of a pain in the rear to get set up, and Mac OS X has a well-developed UI. It's a great solution - if you're a power user/unix guru, you've got access to that when you need it, but for day to day stuff, the Mac UI is intuitive and nice to work with. It also doesn't hurt that every Mac comes with a developer suite included.

If I had my way, Apple would officially support people building their own machines to run Mac OS, and game developers beyond Blizzard and a select few others would support Mac, and then I'd only need 1 computer. That, sadly, is a pipe dream on all fronts.