This is not a supported configuration nor am I recommending you to try this at home/work HOWEVER I'm happily running Virtual Server R2 (pre-release version) on Windows Vista Beta 1 - I'm runnning the beta that's available from the MSDN subscriber centre. I haven't yet managed to network my virtual clients with the external interfaces of the machine - having some problems doing so but otherwise it seems to work OK. I'll let you know via this blog of any problems I encounter.
I ran a risk last night by presenting an entire TechNet event from the machine (including projecting the screen which is something some people have struggled with) without incident - I did have my XP SP2 system powered up and standing by incase there were problems :-)
Incidentally you don't need Windows Server R2 to run Virtual Server R2 just before anyone asks.