Netkas got 6970 going a couple weeks ago.
Unfortunately, it has a lot of "buts" right now.
You have to use his patched 10.7.2 driver and then you need to come out of a restart from Windows into OSX to have the 6950/70 work. Even when running at their best, they have a few more issues with Photoshop.
Also keep in mind that even when he (or Apple) gets a solid fix, the 6970 (like GTX480/580 and some other ATI cards) will require an 8 Pin power connector, which does not exist in Mac Pro.
And while an adapter can be used to change 6 pins to 8...it can't add the ability to carry the required extra current.
The 6950 may end up being biggest & best ATI card in near future for easy Mac Pro use.
The EFI flash he has developed so far is EFI64 only. It is likely that there will eventually be an EBC one for ALL Macs, but not today.