Microsoft Virtualization Benefits report
Microsoft has stood up a really useful site - http://www.hyper-green.com - that can help you estimate your electrical and energy cost savings based on the number of servers that you virtualize. It has a nice interface and actually spits out some useful numbers. It does not really take in to consideration the cost of the virtualization frame (servers, networking, storage), but does provide a userful starting point and some hard numbers to help justify virtualization. Thanks to Doug Fidler for sharing this with me!