Microsoft has published a Windows Server 2008 white paper today (DOCX format) that might be of interest:

In Windows Server 2008, Microsoft is introducing new features and technologies, some of which were not available in Windows Server 2003 with Service Pack 1 (SP1), that will help to reduce the power consumption of server and client operating systems, minimize environmental byproducts, and increase server efficiency. This document describes some of these features and technologies.

Executive Summary

Energy-efficient computing is becoming a priority for businesses and organizations seeking to control costs and reduce their impact on the environment. One way for an organization to reduce power usage is to implement an energy-efficient server infrastructure that facilitates management and allocation of power to computing resources as needed.

Microsoft Windows Server 2008 has been designed with energy efficiency in mind, to provide customers with ready and convenient access to a number of new power-saving features. It includes updated support for Advanced Configuration and Power Interface (ACPI) processor power management (PPM) features, including support for processor performance states (P-states) and processor idle sleep states on multiprocessor systems. These features simplify power management in Windows Server 2008 (WS08) and can be managed easily across servers and clients using Group Policies. Even more significant power savings are made possible by Hyper-V, the hypervisor-based virtualization technology included as a server role in Windows Server 2008. Hyper-V makes it possible to consolidate servers onto a much smaller number of physical machines, significantly reducing power consumption without unduly sacrificing performance.

Update: The download link above has changed somewhat thanks to a slightly revised version of the document. Make sure you've got the latest copy!