This is a good question.
In my mind, the issue has never been do I choose a Mac *or* a PC (or Linux!!!). If I were forced to choose, then I say Mac OS X, hands down.
I think the question for many of us is: Can I have a Mac *and* a PC? As computer prices lower, and as USB and FireWire devices become increasingly cross-platform, I think many professionals should consider having access to one of each.
It's still a Windows-centric world (although OS X may have some impact on that), but let's face it, Mac OS X will soon become the OS we've been waiting for. And if you could see the talent in our open source community already pushing the envelope on the Mac, you might agree that we haven't seen anything yet.
(Oops, sorry for the digression :)
I'm a Web professional, and I like having the various platform/browser combinations to make sure my stuff looks good everywhere. So for the time being, I vote for both.
PS: Guess which laptop I take home each night and which one stays at the office?
PS2: XP installs easily *if* you start with a clean drive. Beware trying to upgrade over an existing Windows OS ...