If you have Exchange 2007 running in a Hyper-V guest (so it might be Exchange 2007 on Server 2008, SBS 2008 or EBS 2008) and you perform a VSS backup from the host machine, then Microsoft have officially stated that they support the Exchange backup in this scenario.
The critical bits of information are on this page: http://technet.microsoft.com/en-us/library/cc794548.aspx
under the section Backup and Restore Considerations
VSS backup of an Exchange guest virtual machine from within the root machine is supported when Hyper-V virtual hard disks (VHDs) are used.
This means that backing up your Hyper-V guests by imaging the host machine is supported by Microsoft.