Bitcoin is using +90% CPU power using Ubuntu 12.04?
Using bitcoin-qt you can go into Helps->Debug Window->Console and then type 

setgenerate false


By default it should already be false so my guess is that you have a fast internet connection and that your computer receive a lot of blocks in a short time. Your computer then needs to check all the ECDSA signatures for the transaction and this can consume a lot of CPU.

I had an overheating issue like that when I was mining litecoins with my laptop. I installed the cpufreq package to decrease my cpu frequency about 30% and everything was fine then.

If you have a dual-core processor you should type something like this in your terminal.

sudo cpufreq-set -c 0 -u "1.8GHz"
sudo cpufreq-set -c 1 -u "1.8GHz"