FUD Zilla reports about a demonstration AMD gave at Computex today. They say the quad-core Barcelona processors are much more power efficient than the current dual-core processors:
AMD had set up what was apparently two identical systems, one with two dual core Opteron's and one with two quad core Opteron's. The quad core system drew 214W at full load, while the dual core system drew 259W. This varied slightly, but overall the quad core system was some 50-55W more power efficent.
In reality this means that you get twice the computing power in your servers without having to worry about any increase in power, which will make a lot of people happy, as there are already problems in most server rooms with new servers drawing way too much power.