Nintendo has released photos that confirm that the company has opted to go with an MCM CPU and GPU packaged on the same chip for the Wii U. The MCM package contains both the Multi-core CPU, as well as the GPU and a large section of one-chip memory.
The MCM concept allowed Nintendo to achieve a couple of things. It allowed for a cost reduction because everything is on one chip and they were able to speed up data exchanges between the two chips as they are on the same package. Of course, this has the added benefit of lower power consumption.
According to Nintendo, the Wii U produces three times the heat that the Wii console did, which made cooling a bit of a challenge. The Wii U uses a fan to draw air in from the side which is directed out of the back of the unit with a small fan. While the heat sink is rather large, it looks to be a very effective design.
See the pictures here.