When customers stop accepting it as an excuse to drop support.
We had the problem on an install once, someone blabbed about it being on a VM. Before they would give us any further support we had to secure a hardware server, install it there, and call back when we hit the problem again. They didn't care about my "Told you it wasn't the VM." The purpose was apparently to wear us down to the point where we would quit calling support.
So the moral of the story is: Before buying anything ask "Do you support it on VMWare/Virtual Server/...?" If not, don't buy it. If they lose enough sales due to lack of VM support they will eventually sign on to it, and at that point they can't just blame the VM and stop supporting you anymore.
LOL We are... errr might be, running several of our servers as VMs covertly. The app vendors never know the difference, as they always remote in for support. Of course, they could easily tell if they looked around at certain things within the OS. The network adapters make it a dead giveaway.