There's an interesting article recently published by Mike Otey which leads you through building a virtualized Windows Server 2003 cluster. You will need to be a paidup member to view unfortunately. Thanks Adam for that link. You can find similar information on RoudyBob.NET's excellent blog and also on microsoft.com. As if that's not enough, there's also this on-demand webcast which I think I've mentioned before [sorry ;-)]. So, no excuses for not having enough information at your fingertips if this is something you want to experiment with.
Mike will also be holding an online chat on 25th May at 5PM UK-Time for the "Straight Story on Virtual Server" which will interesting. He has billed it as "Here's your chance to ask him your questions about Virtual Server and get the 'from the trenches' details on how it works and what problems it can and cannot solve. Michael gives you the straight story without pulling any punches."
Sadly, the timing doesn't work for me due to me presenting at the Technical Roadshow next week in Birmingham.