Just to disagree with everyone in the thread (bar the original poster)

:
I think it's possible, for several reasons.
- It looks like upgrading to Vista might force a lot of PC users to buy new PCs, or upgrade RAM/disk/processors etc. to fully support it. If so, the inconvenience of switching to OSX mightn't be much more than the inconvenience of switching to Vista. If Apple are ever going to do this, do it now as Microsoft are preparing a major, "disruptive" release.
- OSX on a PC mightn't be as seamless as OSX on a Mac, but that's not the comparison new users will be making. They'll be comparing OSX on a PC to Windows on a PC, and OSX might well be a lot better. Hell, I'm using Windows (via Boot Camp beta) on my Mac, and it's superb. Why should OSX on a PC be all that different?
- Apple has expanded its software portfolio greatly since the last attempt at a Mac clone market - they now have significant other sources of revenue other than hardware: iTMS, .Mac, QuickTime Pro, iLife, iWork, Final Cut Pro/Express, DVD Studio Pro, Motion, Shake, Aperture. If OSX was available to the entire market, it would expand the potential market for these apps enormously.
- A lot of OEMs would love to see Microsoft get some competition, which would strengthen their negotiations with them.
- Apple has designed the MacPro to be more expandable than any previous Mac, faster, and it is priced very, very aggressively. If I didn't know better, I'd think was focusing on making it a very competitive machine against the best the PC market can offer, which makes sense if OSX would be able to run on PCs in the near future.
Apple would want existing Mac users to keep using Macs (familiarity, best compatibility and integration), while getting as many PC users to install OSX; and hopefully eventually make the move to Macs too.