I have 2 servers running virtuoso in a cluster. The biggest drawback of Virtuoso is the VM's have to be the same OS as the host. I started out with 2003 64 bit so we can use all 16 gigs of memory in each server, however many of the vendors would not support their product on a 64 bit platform. So I bought 2003 32bit Enterprise. The physical to virtual would still not work even from a 32bit enterprise server. Other issues that I've had have been the backups. You can back up the VM, however multiple times it has froze. When this happens you can't migrate the remaining VM's to the other server. The only way around this was to reboot the entire host system.
I do not recommend Virtuoso for anything more then interface servers or any other server does not require you to disrupt alot of people if it goes down (exchange or terminal servers would be a nightmare)
