My little sub-notebook has something called a "Crusoe" chip placarded at 600MHz but in reality it's nearer 400MHz of other manufacturers. Something to do with the limited instruction set built into the chip so that it has to work harder to get the same result as an Intel or AMD.
But the resulting battery life is improved. Bit of a Catch 22 init? Make it run slower to make the battery last longer doing the same tasks that a faster processor would do in a shorter time...
