I can recommend you check out this just released white paper. As Steve Goodman already covered here, it’s now supported to virtualize the Unified Messaging role as long as you run Exchange 2010 SP1.But it gets even better. Many admins have been confused to say the least in regards to whether they could place virtual Exchange 2010 Mailbox servers part of a DAG on a virtualization infrastructure such as Hyper-V where HA is enabled. Until now this has been a “no go”. However this is no longer the case as long as you do as instructed in the “Hyper-V Failover Clustering” section of the white paper.
Technology Architect/Writer/MS Vendor
MCM: Exchange 2007 | MVP: Exchange Architecture