Microsoft's Environment site has a post about Windows Server 2008 Enabling Energy-Efficient Performance.

Server Technology Evolves
Microsoft Windows Server 2008 is designed with energy efficiency in mind and provides a number of new power-saving features:
Group Policy, which manages Windows Server 2008 power-management features across multiple clients and servers.
Updated support for Advanced Configuration and Power Interface (ACPI) Processor Power Management (PPM) features.
New support for processor performance states (P-states) and processor idle sleep states on multiprocessor systems.
New Hyper-V technology, which makes it possible to consolidate servers onto a smaller number of computers. This hypervisor-based virtualization feature saves power without sacrificing performance.