The folks at Solution Center Ltd., whose job is to help enterprises align their IT infrastructure with their business needs, have a nice write-up on VMware and Cassatt Collage XVM. They do a good job of describing at a high level how Collage XVM works with and how it extends native VMware resource optimization capabilities.

And while I'm in a linking mood, and along the same topic, you might also be interested in Rob Gingell's (Executive VP and CTO of Cassatt) podcast on virtualization in the data center. (You'll have to register with an email address to download the podcast.) While Rob's discussion is broad in scope, you can see how XVM is a piece of the larger solution in which Collage treats VMs as a utility service.