I have this issue also with two AMD 7970 cards in my Win7 system. When I open a project, a pop-up message informs me that hardware acceleration is not supported on this system.
In project settings > general, the video rendering and playback option is greyed out with the selection "Mercury Playback Engine Software Only" as the only choice.
I have the most up to date drivers for these cards.
UPDATE:
I restarted PP and everything worked fine and I was able to select hardware acceleration. Go figure.
I checked in Task Manager and there is an Adobe process running called GPUSniffer; I wonder if perhaps it didn't load properly the first time.
