Shawn Riesterer has a nice post on Build Machine Virtualization. The build team where I work is dipping their toe in the pool. I have jumped in myself. In fact this weekend I will be making the build servers for CI Factory, Doubler, and Refly publicly avalible. I built them in a virtual machine on my laptop and will transfering the image to my fathers web server on Saturday.
The dev team at work has started maintaining virtual images for all supported versions of our flagship product. This helps keep how it takes to transition between branches down as well as getting new developers up, running and productive.