As you probably know, the main risk is with the real time type of stuff - like voice streams. In a well configured, not overloaded environment it should be fine. However a VMWare server running on Windows, with a single nic, etc etc is likely to have issues.
If the customer is willing to do whatever (add hardware, etc) to their VMWare environment, or let me evaluate it first, I'd be more than happy to give it a go. Communication would be key.
-Chak
