I finally got around to upgrading my desktop to Windows 7. After backing up my Vista desktop to a network share, I reinstalled. Upon reinstalling, I went to mount the VHD created (The Lazy Admin has an easy procedure documented), but I kept getting a cryptic error dialog "The version does not support this version of the file format.".
It turns out that you can't mount a VHD from a network share - it has to be copied to local storage before you can proceed. Shame on Microsoft for reusing error messages outside their context.