I had the same problem. Examining the console log showed that I needed to agree to some terms of service, but no terms of service showed up to agree to. On the iCloud panel I signed out and then signed in again. I had to agree to remove a bunch of stuff to sign out. When I signed in again the terms of service panel showed up and I was finally able to log in. Hope this helps!
Some error text from console:
ErrorID = "MOBILEME_TERMS_OF_SERVICE_UPDATE"
description = "This account needs to accept the ToS"
(even though mobileme is gone, I guess the errors haven't been updated)