I've read a couple of posts about providing VMWare-style "undoable" disks (which would be a great thing)... and wanted to point out that the best solution I can think of would be to take advantage of the DiskImages framework: It's shadow function provides undoability for any disk image, it supports sparse and compressed formats, and (best of all) they can be manipulated seamlessly with a host of tools provided with OS X. Take a look at http://developer.apple.com/documentation/Darwin/Reference/ManPages/man1/hdiutil.1.html to get an idea of all the stuff you can do with it. So yeah, use the wheels Apple already invented... in this case, they're really nice dubs.
