Ben Armstrong is a Program manager on the Virtual machine team here at Microsoft. He has posted another solution for backing up virtual machines more quickly and efficiently that putting them in to saved state and copying around all of the files.

Essentially his suggestion is to convert each Virtual Hard Disk to Differencing Disks, thus allowing for the original vhd file to be backed up at leasure without requiring down time whilst the files are being copied as before.

The full description and script can be found here:

http://blogs.msdn.com/virtual_pc_guy/archive/2005/03/04/384928.aspx

Enjoy!