MobileMe Stuck on 'Registering Computer...'? Try this
I was having tons of trouble getting my brand new 11" MacBook Air to get MobileMe Sync set up, and it kept getting stuck with 'Registering computer...' either when I checked the 'Synchronize with MobileMe' checkbox or when I clicked 'Advanced...' and then 'Register Computer'.
Since I've subscribed to iTools, then .Mac, and now MobileMe (soon iCloud) since 2000, I figured this may have something to do with the fact that, after all these years, my default AppleID would change to @me.com (rather than @mac.com, as it has been for years).
The steps you should try before giving up are as follows: