Not officially supported, but I have been running 2 BES servers on Microsoft Virtual Server 2005 for about a year now with no issues. For performance reasons (meaning only so much RAM that can be allocated from the host server), we have moved the database to SQL Server.
I have my test 4.1 BES on VM ware. Only reason it wouldn't keep it there is that when MDS, attachments, IM and everything else is running, it is quite memory intensive. When I move my 4.1 to production with ~400 people I'm going to throw 4GB of RAM at it. 4GB on a VMware guest defeats the purpose in my book.