It's a very common process to reduce heat and power consumption. For instance all the previous iphone and ipod touch underclocked their processor and the new HTC incredible underclocks. Just about every embedded device underclocks as a higher speed processor often runs more efficiently at a lower speed than a processor designed to run at that speed.

It could be that for short bursts (which usually come immediately after user interaction), they use the full clock speed, but for long running computations, they throttle down.