Well, I would say it requires a BIOS-supporting flash. I've flashed a lot of Mac/EFI firmwares onto older cards (Radeon 9700 Pro, GeForce 6, etc).
But what someone mentioned in another link about newer Mac hardware also supporting BIOS due to Boot Camp compatibility is interesting. Obviously it supports something BIOS-related as Windows boots up just fine on Macs, but it still may be a special, proprietary BIOS-ROM that doesn't work on "real" PC/BIOS systems.
Then again, through the magic of Hackintosh, x86 Mac OS X can obviously boot and run on a BIOS-only video card.