NO kidding! My 3 core Phenom desktop was powering itself off every once in a while (Kubuntu 11.04). I opened it up and boy, was it hot! I took the opportunity to clean up the fans (the had lots of dust and lint), the ventilation grids on the box, etc. Reseated the CPU cooler. I am not really sure how much of this was related to the new kernel. It's running 10C cooler now.
Anyways, the two first lines in powertop seem suspicious (and consistent with what Michael was saying)
I'll keep digging ...
42.2% (986.5) [Rescheduling interrupts] <kernel IPI>
19.0% (444.5) [extra timer interrupt]