InfoWorld writes that Microsoft released a study that Windows Server 2008 cuts power consumption by 10%.

With electricity prices continuing to skyrocket and processors getting ever hungrier for power, it was only a matter of time before Microsoft (NSDQ: MSFT) chimed in with claims that its latest software can cut energy bills. A Microsoft white paper released this week asserts that Windows Server 2008 can cut power consumption by 10% compared with Windows Server 2003 out of the box, and much more if running virtualized.
Microsoft compared power consumption between two installations on the same server with two dual-core processors and 4 GB of RAM, one running Windows Server 2003 R2 Enterprise x64 Edition with SP2 plus hot fixes, and the other running Windows Server 2008 Enterprise Edition, with a hard drive formatting in between.
The company found that Windows Server 2003 used as much as 10% more power despite only being able to deliver 80% of the maximum throughput as its successor. Microsoft attributes these improvements partially to power management features that Windows Server 2008 has enabled by default, like the automatic adjustment of processor performance based on workload.

But, what happened to the 20% Bill Laing discussed with Mary Jo Foley

We've done power management by default in Longhorn Server. And we think average machines will see maybe 20 percent reduction in power use. You kind of slow the clock down when it's not busy. And it's dynamic enough that you can literally slow the clock down across a disk I/O. If you've got nothing to do while you're doing a disk I/O, it actually drops the power use for that short period of time. It's not like sleeping [for] the laptop; this is really short, what they call P-state for processor state.

This continues to support one of my beliefs that power management needs to exist in a bigger picture than the server itself. What is needed is power management to be designed across systems. This part of what the guys at Cassatt Software have done.