Microsoft released a brochure for Data Protection Manager 2010 and how to use it to backup Hyper-V.  Here’s the readers digest version:

• You can install an agent in the VM, like you would with a physical box.  That’s a good idea for selective backup of things like SharePoint, SQL, Exchange, etc, where you want to do granular backup/recovery of applications.
• You should backup at the storage level.  Don’t think of it as the host level.
• For non-clustered hosts, you install an agent on the host and it backs up at the storage level using VSS.
• For a cluster, you use a storage VSS provider (choose your storage wisely) and it backs up the CSV(s) using VSS … that triggers VSS in the VM’s and the VSS writers in the VM’s guest operating system for a nice clean backup.
• It’s best to install DPM 2010 on a physical box.  This means you can enable the Hyper-V role.  This reveals DLLs that allow DPM to access the contents of a protected VHD and perform item level recovery from it.
• Only use passthrough disks for DPM storage if you install it on a VM.