Microsoft is really eating their own dogfood. They just annonced the complete migration of both MSDN and TechNet, two of the most popular web sites in the world, are now running on virtual machines (Microsoft Hyper-V).
Microsoft kept the back-end database on physical boxes, but moved 100% of its IIS7 frond-ends on Hyper-V RC0 VMs with 4 virtual CPUs and 10GB RAM.
The virtualization hosts (no mention of the brand obviously) are powered by 2 Intel quad-core CPUs and 32GB RAM (2GB are reserved for the Windows Server 2008 parent partition).
Read more about this interesting case on virtualization.info.