Does overheating cause a computer to slow down?

Most modern processors do reduce their clockspeed when they get warm to try and cool themselves. I think it all started off with laptops and having dynamic fans and powersaving etc, but as they all pretty much run off the same set of chips then they've all started doing it.

I used to have a fanless laptop that ran at 1.1ghz, unless it was warm and then it dropped down to about 300mhz until it cooled down!