What are you doing these days? Not seen you around 'ere for ages. Sorry to hijack the thread haha.
On topic then, I too have had this problem, and I ended up doing what you did. Deleted all history and unused materials and anything I could clean up. Exported the geometry into a file and started a new scene, re-import the geo. I thought it was an uggly workaround, and as often with those, rather than wasting more time trying to understand what was really going on I ended up with a hack wasting time setting everything up again (render globals, reassigning materials etc).
Temporarily solved but I still have no clue why it happened or if it could of been fixed with a far easier solution, or something simple but not yet clear to us (like the click of a button in some window or sommit silly).
The worst part is, such problems have a tendency to repeat itself. So if anyone has an insight to what is really going on when this happens, please do share.