Personally I would call anything over 80 degrees C too hot, but 90 isn't in the danger zone just yet.
I should have specified what I meant by danger zone. Don't expect a crippling failure at 90 degrees in the short run, but it's going to shorten it's life expectancy in the long run.
When I used openCL on my GTX460, it mined fine at ~60 MH/s, but when ever I stopped mining, it would crash my computer.. I switched to rpcminer-cuda with no performance loss and everything works fine.