Over the past day on my Windows 7 x64 workstation I've done some updates. Specifically:
I was just doing some OpenGL testing with Photoshop CS5 12.0.4 x64 and I noticed that under some conditions I saw Photoshop drop out of OpenGL acceleration. Specifically, when I opened a Canon 5D Mark II image through Camera Raw I saw subsequent operations stop using OpenGL acceleration even though the checkmark remained in the [ ] Enable OpenGL Drawing box in Edit - Preferences - Performance.
What I did to determine whether OpenGL acceleration was enabled was this: Select the Zoom Tool, then click and hold the left mouse button on the image. When OpenGL is enabled, I see a smooth increase in zoom. When it goes disabled, I see only a jump in zoom level after letting up the mouse button. Also, the [ ] Scrubby Zoom box gets grayed out. As I mentioned, even in this condition, a check of Edit - Preferences - Performance still shows OpenGL enabled.
Just as a control, I saved the converted file as a PSD, and every time I opened THAT file (with a fresh copy of PS CS5 running) and did the same operations I could not reproduce the failure. The difference being I did not run Camera Raw.
Since I wasn't specifically looking for issues with Camera Raw, I am not sure that the problem occurred every time I did run Camera Raw. I do know that when I open images from my own camera (40D; I do not own a 5D Mark II) that the problem doesn't seem to occur. Notably I always open my image from Camera Raw to the largest possible pixel size - 6144 x 4096, as I did with the 5D Mark II image, so it's not an obvious size difference that's leading to the issue. Given other comments of late, I'm wondering if it could be a specific issue with conversions of files from 5D Mark II.
Nor am I sure whether any of the above updates caused it - it might done this before; I don't regularly convert 5D Mark II images. I DO think I would have noticed Photoshop reverting to GDI operation before, since I just noticed it pretty easily, but I'm not completely sure of that either. I do use OpenGL-specific features (such as right-click brush sizing) pretty often.
I'm trying now to find a set of steps with which to reliably reproduce the problem now, and will advise.
It's not consistent. Sometimes OpenGL remains enabled even after opening the 5D Mark II file through Camera Raw. In fact, I haven't been able to reproduce the problem again.
Figures. Now I can't reproduce the problem at all, even after an hour of testing. It probably had nothing to do with Camera Raw.
I wonder if there are latent GPU status indicators and config values stored by Photoshop based on its specific environment that might not be right for the first run (or first few runs) after a display driver update. Hm....