Exchange Server 2007 SP1 is supported running on Hyper-V or any other hardware virtualization platform that is validated as part of the Server Virtualization Validation Program (SVVP), program provided Exchange is running under Windows Server 2008.
Hyper-V has a number of host-level high-availability solutions such as Quick Migration and Live Migration, which make the virtual machine (VM) highly available in a manner invisible to the actual VM. These host-level high-availability solutions are not recommended for Exchange 2007 because these solutions provide only a loss-less failover in planned (scheduled outage) situations where the administrator manually moves a service from one node to another and even then Exchange is not aware of the move.
In a disaster situation (unscheduled outage), information on the guest OS may potentially be lost, and when Exchange is cold booted, the Logfiles are replayed as if the system had been shut down in dirty state. The recommendation from the Exchange product team is to implement high availability in the actual guest OSs through technologies such as Cluster Continuous Replication (CCR) and Standby Continuous Replication (SCR), which also provide multiple copies of the data and avoid data loss through the use of the transport dumpster.
You can see Microsoft's full recommendations at http://technet.microsoft.com/en-us/library/cc794548.aspx.