Problem Solved....so far anyway.....
After stripping and rebuilding my A500 I can tell you that, so far as I can tell, I have solved the problem.
The rear camera module seems to be very loosely attatched to the motherboard by a pushfit connection (like laptop dial-up modems are connected).
The process went as follows:
1) I removed the camera module
2) I checked the connections were not damaged
3) I checked that the plastic cradle that supports the module on the motherboard for fit and position
4) I replaced the camera module, making sure it was CORRECTLY ALIGNED BEFORE pushing it back down
5) As an extra precaution before reassembly, I taped the module down to the board from the socket side, just in case it ever decides to dislodge again.
6) Reassemble the tablet.
Whether it was the strip and rebuild that corrected the problem or the NECESSARY reset via the tiny reset button (it wouldn't reboot without resetting, probably due to the battery removal on strip down), I'm not sure but the problem seems to have rectified itself for now.
If it ever happens again, I'll repost here to confirm that it was the hard reset that helped.
For anyone that has problems with their A500/A501, here is a link to the service manua
l hosted online