I was reading this article at Sys-Con about how Vanderbilt University is using server virtualization to save energy and felt this paragraph really drives the point home:

It's carpooling for computers, and its impact at Vanderbilt is just beginning. It's a growing trend in the industry as demand for computer power continues to increase, and Vanderbilt is helping to lead the way among the university community.

The emphasis is mine but that phrase really does get the concept across.

The full article is here, which goes on to discuss Vanderbilt's results:

In the past year, ITS officials estimate that Vanderbilt began saving 20,575 watts per hour because of server virtualization for 35 percent of the servers they manage. Efforts are underway to increase that to 50 percent soon, and 75 or 80 percent as time goes on.