[russell_h]

You should also check out Ganeti: http://code.google.com/p/ganeti

Its basically a tool for managing virtualization clusters, with a ton of nice features: live migrations, redundant storage over DRBD, support for KVM and Xen (I think support for a few other virtuualization/containerization systems is in the works too), scriptable deployments and an HTTP API. It is, as I understand it, used internally at Google (they developed it) for a few things, and judging by the mailing lists there are a number of other good sized deployments around as well.

In a few hours you can set up your own internal "cloud" with Ganeti, and its fairly easy to scale later as the need arises. Bring a new node online, get on the Ganeti master node and tell it the hostname and root password, and Ganeti will SSH in and take care of most of the setup for you.