Like having multiple virtual hard disk (VHD) access, having the ability to UNDO disk writes adds a lot of flexibility to a virtual machine environment. It enables a variety of scenarios -- for example, testing -- where using a virtual machine might result in changes to the contents of VHD that you want might want to be undone before the next round of testing. This can also be accomplished by simply making copies of your VHDs and renaming those copies to the original VHD name when you want to start with the original one again. But that can be pretty tedious.
For David Berlind's write-up on The Virtual PC 2007 User Interface, see his post
in ZDNet's TestBed blog.