From the release notes for ADE 1.7.2, which has just been released:
Please note that as of 15 December 2009, Adobe shut down the activation server for Content Server 3 (the predecessor to today’s Content Server 4). In consequence, users who never migrated their old PDF eBooks (purchased prior to September 2008) can now never migrate them.
Just another example of the utter Fail that is DRM.
Here I am, dragging this thread back from the dead