As Paul Cunningham from Exchange Server Pro correctly points out in this post Microsoft does not support Live Migration or vMotion of Mailbox Servers if they are part of a DAG (Database Avalaibility Group). Paul also mentions something which Scott Schnoll talked about at Teched last week, that Microsoft does not support high availability solutions in the hypervisor, i.e. VMware HA. Refer to the following line in the system requirements:
"Microsoft doesn't support combining Exchange high availability solutions (database availability groups (DAGs)) with hypervisor-based clustering, high availability, or migration solutions that will move or automatically failover mailbox servers that are members of a DAG between clustered root servers. DAGs are supported in hardware virtualization environments provided that the virtualization environment doesn't employ clustered root servers, or the clustered root servers have been configured to never failover or automatically move mailbox servers that are me ...