OMG I can remember these being demo'd at Comdex in the mid 90's.
A TEC is basically a ceramic wafer with wiring running through it. You run a current through it and one side gets hot and the other side gets cold. Simple idea, not so simple to implement. The cold side gets colder than ambient temperature thus causing condensation. You would need to insulate the CPU area as if you were using phase change cooling. Also, the hot side can get really hot and needs something to cool it or it will overheat and kill the TEC unit - after which your CPU wouldn't last too long. Looking at the Swiftech stuff, their TECs are cooled by water. This makes the Swiftech kind of a cheaper version of a phase change cooler, perhaps not quite getting as cold but definately getting below ambient temperature which no water or air cooling can do unaided.
Also as mentioned, TECs eat up power. This power drains off as more heat on the hot side, so it isn't as if it drops 20º on the cold side and goes up 20º on the hot side. It'll drop 20º on the cold side and go up 40º on the hot side. I'm just pulling numbers out of thin air to illustrate the point.