I thought for a while it was some rogue application eating up CPU, and sure enough Mozilla, Chimera, and NetNewsWire all seem to waste cycles when they're not obviously doing anything. But it happens even when I kill them. And I've just realized that it only seems to happen when I'm plugged in with the power adapter.
Looks like it's related to the "processor performance" setting under Energy Saver. I wish I knew more about what the "reduced" setting means. How slow is it? Is it always reduced, or does it just pull back the throttle when the temperature is too high? That's the setting I would like to have for use when the power is plugged in: full speed ahead unless things get hot, and then pull back to avoid having to run the fan at high speed. (For most things I do, I prefer quiet to blazing performance.)