Wow. Can't believe it's close to 3 months since posting - just shows how busy life can get sometimes. Anyway, I thought I'd share this script which will tell you how many domain-joined Virtual Server hosts are currently running in your organization. It takes advantage of the SCP (Service Connection Point) marker in Active Directory which was introduced in Virtual Server 2005 R2 SP1 (currently in Beta - not too much longer to wait before release).
Either download a text file with the contents here and rename it, or copy the above code into a file "FindServers.vbs". Edit the call to "DoQuery" at the top to put in the appropriate domains for your organization. If you want a list of the machine names as well, uncomment the code-block starting "oRecordSet.MoveFirst" and ending "Loop". To run the script, from a command-line, type "cscript FindServers.vbs".