[Kevin Patrick]"Are they running some standard Windows NVIDIA cards (as in not For Mac) with just the correct NVIDIA driver?"
They are flashing standard third party Windows/Linux NVIDIA cards with a custom ROM that is EFI-friendly. Macs use EFI instead of BIOS to initialize hardware, and almost all third-party GPUs will only initialize with BIOS. That is why you can't just buy any old GPU for a Mac Pro (that and drivers, but more about that below).
[Kevin Patrick]"One of the posters refers to using a GTX 570, nothing modded. "
The NVIDIA GeForce drivers recently released for Lion 10.7.3 reportedly will work with GTX5XX cards without any mods, though they do not show the grey Apple boot screen- they only start to work once OS X loads the drivers (login window). Then as noted in the thread I linked, they apparently don't use all the available PCIe bandwidth. Getting an EFI-flashed card and installing the NVIDIA drivers gets you complete (if unofficial) GTX5XX support. At least according to the guys in that thread I linked.