Microsoft announced today that the UM role can now be virtualized with Exchange 2010 SP1. Before today Microsoft would not support UM on Hyper-V, VMWare, or any virtualization technology. In addition to announcing this support Microsoft also released a new best practices white paper on virtualizing Exchange 2010 SP1 with Windows Server 2008 R2 Hyper-V. Finally Microsoft published multiple case studies on customers who have virtualized their Exchange environment.
For more details see: Announcing Enhanced Hardware Virtualization Support for Exchange 2010 on The Exchange Team (You had me at EHLO) Blog