If you haven't noticed, most computers with Linux installed by default aren't easy to come by. The vast majority of the time you have to go out of your way to get one, and they are rarely any cheaper. In fact, Dell XPS m1330's are routinely more expensive with Ubuntu installed. The exceptions here are the netbooks, of course.
I'd wager that WAY more XP/Vista boxes get reinstalled with Linux than the other way around.