Congratulations for this great and very useful idea!
I wanted to suggest a Read-Only mode for the VHD file, such that when the system is restarted, it can automatically return to a previous known state and discarting all changes that end user does. very usefull feature for education, labs, thin clients, etc... of course, once the vhd has been customized and once working, there could be a suitable way to enable this "lock down" feature.
thanks!
Another idea would be to have it streamed thru pxe or gpxe so the clients could be diskless. Nevertheless for win7 / 2008 probably doing this could have long transfer times and slow performance... but this could be a very interesting option for OS streaming