I came across an article from Express Computer today on environmentally friendly data centers. I forget exactly what I was searching on when I came across it but it caught my eye an was a good read.

On the topic of virtualization, the author notes:

Virtualization and adoption of multi-core processors may also significantly affect the type and quantity of equipment to be installed in a data center.

and

Consolidation/optimization: Optimize by consolidating as much as possible. Virtualization can be used to consolidate servers/applications; use a quad processor server than two separate dual CPU machines (think of cost and simplicity before virtualizing). Use power management tools. Use the hardware to the maximum since the same power is consumed even if server is running at lower utilization levels.