Power saving on servers is a hot item. VMware is having a feature called Distributed Power Management which was experimental in ESX 3.5 and now fully supported in vSphere 4. When this feature is enabled vCenter will automatically adjust the number of running hosts depending on demand. During off hours virtual machines will be consolidated to a reduced number of servers using vMotion. Remaining hosts will be shutdown. In the morning, when demand increased, hosts will automatically start-up again.
Microsoft added core paring and P-state feature into Windows Server 2008 R2. Core parking will suspend one or more running CPU-cores when demand is low. Using P-state the capacity of the CPU is reduced by lowering the clockcycles of the CPU.
About every blogsite mentioning core parking including some documentation published on microsoft.com state core parking and p-state can be controlled using Group Policy (GPO) settings. But nowhere on the internet is explained which settings exactly.
After some reasearch it turns out there is no GPO setting to control core parking and p-state. Instead, control must be done using the POWERCFG command. Another option is to use WMI.
More information on this can be found in the Performance Tuning Guidelines for Windows Server 2008 R2
Information on Power Management in a whitepaper called Power Policy Configuration and Deployment in Windows
Microsoft presentation on Windows Server Power Management
Presentation on remote management of power by WMI