As with anything you should be backing up your Hyper-V machines.  I think that just goes without saying.  Backing up Hyper-V has gotten easier with the addition of VSS and Windows Server Backup (WSB).  WSB is a new feature in Windows Server 2008 that replaces the old NTBackup from days gone by.  It is not installed by default but can be easily added with the Add Features wizard in Server Manager.  Once installed there is one last setting you must configure before backing up the Hyper-V virtual machines.

While Hyper-V does support VSS backups, it is not enabled by default.  I am not sure why but you need to set a few registry settings to get WSB to recognize the Hyper-V VSS writer.  You can enable this with the following two registry settings.

Under HKLM\Software\Microsoft\Windows NT\CurrentVersion\WindowsServerBackup\ApplicationSupport add the following key {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE

Then under that newly created key add a new REG SZ string called Application Identifier with the value Hyper-V.  Once that is done you are ready to go!  To make it a bit easier you can copy and paste the following to a command line (with administrative privileges) and it will get it all set up for you.  You will need to set this on all Hyper-V servers in your organization but there is no reboot required!

reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\WindowsServerBackup\ApplicationSupport\{66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}"

reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\WindowsServerBackup\ApplicationSupport\{66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}" 

/v "Application Identifier" /t Reg_SZ /d Hyper-V

For more information see:

Windows Server 2008 Backup Step By Step