yea, DC.... AC would just turn it into a big heater.
The peltier effect is essentially that when you pass current through the ceramics, heat is transfered, making one side get hot and the other get cold. If you switch the direction the current is flowing, you will switch which side is hot and which is cold. If you were to put AC into a plet, the current flow would be constantly changing direction, so neither side would get cold. Due to the ineffecency of the pelt, the whole thing would heat up a whole lot though.
The effect works in reverse as well, if you heat one side of a pelt and cool the other side, it will generate a voltage and current if connected to something.