Well, it's dissapointing but I've just tried around 7 PC graphics cards in the new Intel Apple Mac Pro and none of them worked in OS X (though I've yet to try them with Boot Camp and Windows installed). Most of the cards were recognised in the System Profiler as display devices and some had the manufacturor name (eg ATI) as well as other info but I could get no display signal from them. Three of the ATI cards displayed the cryptic 'Kernel Extention not loaded' in System Profiler possibly suggesting that OS X itself is hamstrung so you can't use the cards (this is a pure guess as I admit I don't really understand these things at a core OS/driver level).
Cards tested included:
Quadro FX4500 (Nvidia-based)
FX 3500 (Nvidia-based)
I will be checking the cards again with Boot Camp as well as taking some grabs of the profiler info. I'll post more info and pics on my site so book mark it and check back soon...