I accidently opened the rear of the camera after shooting an entire roll (before rewinding). There was the entire roll of exposed negatives, ruined I was certain. But since I needed 4 rolls in my daylight tank, I figured what the hell, there may be something left at the beginning of the roll that didn't get ruined.
What I found was this (and it may not hold true if this happens in bright sunlight): The last 3-4 frames were black, there were 2-3 more that were ruined but only every other frame, the rest of the frames were fine.
The moral is this: If you accidently open the camera back before rewinding - develop it anyway, the bulk of the roll is most likely still okay.