I am working on setting up a hosted Exchange 2010 environment which will consist of: EdgeServer, Client Access Server/Hub Transport Server, Mail Server, and then our DAG. All of which will be added to/expanded as necessary according to growth. My problem is that I am unsure whether to install the Unified Messaging role on its own dedicated server, or on either the CAS/HT or Mail Server. I know that you are able to do both, but have not been able to find documentation on which would be the most effective, or a particular reason why you would do one over the other. The environment is virtualized so an additional server(s) wouldn't be an issue if it is necessary.
Microsoft recommends installing Unified Messaging on its own server. This isn't required, but UM can suck up resources pretty well. It also isn't supported in a virtualized setup. This is exactly what they say about virtualizing UM: