Appleinsider publishes more diagrams of the Liquid Cooling System found in the high end PowerMac G5s.
The new cooling system is apparently necessary for higher density PowerPC 970FX chips according to the Apple senior director of desktop product marketing:
The processor was built using the 90-nanometer process. When you do that, you challenge the power density. You could see the same problem happening with Intel's 90-nanometer chips.