Because I don’t want to copy Michel de Rooij’s blog I will simply make a reference to his blog. Many of us know that the Unified Messaging role was not supported for Virtualization and that the Exchange 2010 Mailbox Role if used in Database Availability Groups was not supported if implemented on top of Hyper-V, VMware or XenServer host failover clustering. So in a Hyper-V environment we were unable to use Live Migration (although I know many who went against that policy).
With the release of a new whitepaper on virtualizing Exchange this now seems to have changed. Michel explains further in his blog.