Ugh, this happened to me, too, while I was on vacation. I had taken some beautiful photos only to discover later that the camera had reset itself from 6 MP to 0.3 MP. Just prior to that, I'd taken a panoramic shot, which is only available at 640x480.
The drop only occurred once; Before and since the episode, the software reset to my higher resolution of 6 MP when switched from panoramic to "single shot" mode. I've not been able to reproduce the problem and now obsessively check to ensure my resolution hasn't inexplicably dropped to 640x480.
I was hoping the camera app takes the photos at the high resolution and then resizes them via the software to 640x480, so that there may be a way to retrieve the "original" higher resolution image or metadata somewhere. Alas, I've found no evidence that is the case.
My phone is rooted, version 4.04, build IMM76D.FK19. I had GPS enabled when it happened.