Ive wondered this also. In theory it should work, since they use a unified driver, but I do not know this to be fact. Obviously you would not get the ADC port, but is that the only thing missing?? I dont know. Anyone else know how the unified drivers work?
There's a flash matrix for Nvidia/ATI cards lurking somewhere around that IDs the results you can expect -- some are not very good (the cards that don't have enough EPROM for a universal flash).
Of course a card that work right now, can be rendered quite unstable with a Apple firmware/OS update and/or a driver update.