MobileMe
I appear to be one of the users afflicted. Existing computers that were syncing fine stopped and don’t show as registered. New computer never before synced also can’t register. Apple Support forums with many user reports – e.g., http://discussions.apple.com/thread.jspa?threadID=2553248&start=15&tstart=0
Waited 1hr 32 min for chat rep who, like all first line agents in every company wanted me to jump through every hoop and blame my computer(s) – my network – my frame of mind. And finally escalated and then admitted it was an Apple problem. Then the solution was “we know, wait a few days, keep trying”. Apple, this is NOT acceptable.