Is the display cable connected properly or is it damaged?
I had something similar happen when my 8800 GT died.
The original card still works? Try it.
You do have both 6-pin auxiliary power cables connected to the card, right?
My 5870 is working fine in my 2009 Mac Pro....
Yes I have it back in. Have a man called "The Digital Plumber" calling in next week. I will update later. Thanks for all your help.
You did try re-seating the card in the MacPro or trying an alternative PCI-e 16x slot ?
If it wasn't properly installed even by the tiniest millimeter it could have created this issue, though like others I suspect it was probably a lemon.