e79

There are some GC bug fixes that address some edge cases.

Their support page makes it sound like they can just de-register iMessage with your account.

http://support.apple.com/kb/ts5185

A few comments here seem to suggest that this is a carrier or cellular infrastructure issue. It isn't! iMessage doesn't route over SMS-- that's the whole point. It routes to Apple's servers, which should be capable of doing a lookup to see if the number still has an associated iCloud or iMessage account.