"as long as there's no control over what public keys we encrypt the message with, the statement that Apple or the NSA can't read the messages is a half-truth at best."

This.

That said, it is reasonable to believe the way iMessage is architected would likely make mass surveillance harder in general, just like widespread use of SSL does, so it is not entirely useless either.

The bigger problem IMO is that they then upload the message database unencrypted (from server's standpoint) to iCloud in the backup process (which is admittedly optional, but effectively on for most users). That, of course, is easily readable, as the celebrity hack shows.