Mitch Denny from Readify has released the first (publicly) version of TFS Build Virtualizer.
Haven't tried it yet, but it seems like a great tool. TFS Build Virtualizer, allows you to have a bunch of virtual machines as build servers running inside Virtual Server. All the machines are stopped and when a build kicks in the machine is automatically started the build is done on the virtual machine (and the machine is stopped when the build is done).
This will allow you to have a separate virtual machine for each build, with projects truly isolated from each other, while at the same having a less demanding server machine.
This might come in handy, we are internally debating if we should use a physical machine for all our builds or if we should use virtual machines.
This might swing the pendulum to the virtual machine choice. :-)
Kudos do Mitch Denny (and he seems to thinking of releasing more tools in the near future :-)
[Update] Mitch has published a Getting Started with TFS Build Virtualizer post