For your information, I have managed to mix the PC BIOS of my card and the EFI part of the MAC ROM.
What I get is the same as a Hackintosh :
- Dual screen not working in Mac OS X : single headed display only (using the DVI port closest to the Logic Board, the other one does not work)
- VGA output & dual screen working in BootCamp (of course, the original firmware is used ...)
Now, if someone with a unsuccessful flash could pm me a link to their original bios, maybe I can make something out of it.
Same thing if one of you has a 4870x2, or a 1GB 4870, and flashing with the dumped MAC firmware didn't work : send me a link to your original rom, and we'll see.
@netkas: Do you think this single display only issue can be fixed using Natit ?