I've been doing a lot of work with Hyper-V and System Center Virtual Machine Manager 2008 for a recent project. Backup needs to be addressed as part of this project. So, I started testing out Data Protection Manager (DPM) and found it to be quite nice. I'd been meaning to take a look at DPM, but never got around to it. Overall it looks like a good and reasonable priced solution for MS shops. There is no agent available for non-Windows servers.
The big thing with DPM is that it's disk-based backup with an option to go to tape. Your initial backup on a server is a full backup, but after that, it's all snapshots. Makes a daily backup go much faster. You can archive to tape on a schedule that you determine. So, cool from that perspective.
Now for backing Hyper-V and Virtual Server VMs you have two options. First, install an agent in the VM and backup the VM like a physical server. This option gives you the most flexibility because you can choose what data to back up and what data to restore. However, you also pay for an agent on each VM.
An alternative is host backups. Host backups are done only at the host level, rather than at the guest level. If the OS and applications in the guest are VSS aware (they have a VSS writer), then a backup can be peformed without taking the server down or pausing it. VSS is used to make all data consistent before a snapshot is taken and the vhd files are backed up.
The upside to host based-backups is lower licensing costs. A single DPM license is installed at the host level and all VMs are backed up with this single license. The downside is recovery flexibility. You can only restore an entire VM. However, if you did need specific files, you could restore the VM to an alternate location and then extract the files you need by mounting the vhd. A pain, but doable and may be worth it depending on the $ you are saving.
For more information about backing up VMs on Hyper-V and Virtual Server, check out this link: http://edge.technet.com/Media/DPM-2007-SP1-Protecting-Hyper-V/