I've also been getting nasty spikes with the latest driver (256.44). Every so often, for 3-5 minutes at a time, all cores will ramp right up to 90-95% CPU.
It's a powerful computer (i7) so I can carry on doing lower-power things but watching fullscreen video, flash-heavy websites or playing games is completely out of the question.
I should add that these spikes are not limited to X. All the CPU monitors show huge load but none show a single process that could account for it. Even with kenel threads showing, I can't make any sense of it.
I'm going to downgrade to 256.35 and see what I can see. I'm on kernel 2.6.35 and in case it's relevant, I've already posted a thread on this on Ubuntu's StackExchange site: