Being another Mac user I may as well throw myself into this impending debate, one hesitating to start like many college students do with a term paper.
Like a majority of long time Mac users, I'm an artist, a graphic designer more specifically and a horrid one to be more painfully honest. Given Apple's history of being graphically more powerful for the type of mediocre perversions of art that I find so satisfying, I chose to purchase this flashy expensive silver brick of a Macbook Pro so I could at least pretend I'm part of the community. Yes, I know I could of spent the same amount of money on a top of the line Vista box, but it's that fundamental difference of being run with Vista that turns me away from that and turns me towards Apple, aside from the shine as if I were some ten year old who confused my ridilin with pixie sticks.
The gap between the two OS is closing, and Microsoft is no longer that bad graphically, but still the same tried and true facts on both remain and for this I stick with Apple and sacrifice any gaming experience. Three to five year games, as some one pointed out, is what we are typically limited to, however this is only because Devs like to make money, and there isn't a lot in the Mac field. I do know that COD4 will be released on the Mac, as well as some other titles, but until then your main friend will be Boot Camp and a copy of an over priced Vista, be it version A or version MK.VII Orange or what ever