But taking the other side of the argument, here are two scenarios where I *wouldn't* use virtualization (one could certainly enumerate more):
1. A production DB server or server cluster.
2. I've had services where I needed to maximize uptime. One option I tried were VMs and being able to move the VM back and forth between hosts. That might cover hardware failure, but I'd still take outages when I needed to upgrade software in the VM. Moving to a traditional HA solution on physical hardware means the outages are now measured in seconds instead of minutes, and most of the time are undetectable by the users.
I also tried services where the HA nodes are themselves VMs, but was less than impressed with operational stability.
When CentOS 6 comes out, though, I'll be interested to see how (2) behaves when it comes time to do a rolling upgrade from CentOS 5 (bring a node down, install and reconfigure C6 from scratch, rejoin the cluster, have C6 take over the services, then upgrade the other node).
Thank god for test environments. And backups.
Devin