I'd like to be able to build a cheap, fast, quad-core machine and have the freedom to put Linux, OSX and Winders on it (or run them all in virtual machines on the same desktop). You can mostly do that now if you're willing to do lots of reading and hackery, if you've got the right parts. Psystar is promising something much easier.
My play time is much more precious than it used to be. I'm willing to spend a few bucks to save a few hours of research and fiddling.
I'm not against spending money on Apple hardware, but the value proposition has to be there. If I can build or buy a fast Core i7 machine for under $1k, it's hard to see spending 2-4x that for a MacPro that's at least a generation behind. Maybe Apple's new MacPro machines (what's taking them so long to update the MacPro to the i7?) will make the point moot for a while.
FWIW.
Cheers,
Scott.