There seems to be a bit of confusion about the benefits of server virtualization, with many tending to focus on cost savings. As a district that has been running a virtual infrastructure for some time, I can honestly say that virtualization is not so much about saving money (although you certainly will) as it is about better resource utilization, more reliability, and greater flexibility.
Better resource utilization
There is no question that most of our servers are doing nothing about 90% of the time. This becomes quite obvious with even a cursory glance at historical utilization data for any given server. It would seem that the obvious solution for this would be to simply run more applications on each one, but the reality of this is that the more apps you install on one OS, the more unreliable it becomes (especially if it's a Microsoft product.) So, what we all do instead is buy a new machine every time we want a new app that we think is "critical," because we want to be sure it has its own sandbox to play in. Don't even tell me you are running anything but Exchange on your Exchange servers!