So I spent the day today playing with ESXi on an Xserve (Early 2009) that I had lying around. After a bit of mucking around, I was able to get an OS X VM up and running on it. Pretty slick really - hard to believe this software is free! ScreenSharing (or ARD) to the VM and you're off and running.
Unfortunately, ESXi can't use the Apple RAID card that my test Xserve has installed, so it can't see the 3x 1TB drives in the sleds, but it can see the SSD and boot off that ok, and I threw another SSD for the datastore in the optical drive bay (using an OWC data doubler).
So far I'm still just getting my feet wet, but I feel like I've taken a step into a larger world! Now to investigate how I can automate backups, etc. but guessing I'll just stick with the current method (of backing up data within the OS) and creating snapshots manually when I'm doing updates, etc.