(Calling dwchang!! Or mneideng if he ever comes here...))
Try this, bum. Heat causes almost all materials to expand in physical size. That includes the metal wires - if you can call them that, being as small as they are - and junctions in the chips. If they expand enough to cross the space between each other, electrical shorts can occur.
I'm not sure how true that is, but it's the first thing that comes to mind. Physical expansion at nano-scales can be deadly.
This article is much more detailed, but probably uses language that only a few of us can understand. (Not to mention that the English is imperfect.)
The two major problems he describes are:
(1) Electromigration...which I suppose means the metal "wires" will essentially start evaporating. Thinner wires means higher resistance means lower performance.
(2) Oxidation...which happens when heat promotes "dirt" - for lack of a simpler word - to accumulate inside a transisitor. A transistor's performance, and there are millions inside a typical CPU, will degrade and possibly fail outright.