For future reference, if it's out of warranty (and assuming Apple doesn't deem this as an ongoing issue*), you can just glue it back into place. It was originally assembled with glue anyway.
* For some other products in the past, there have been a few instances of a particular failure being common enough to warrant free repairs outside the warranty period. Maybe the caseback issue could get similar status, maybe not; it's still pretty early (nobody's AW will be at the 1-year mark for another couple weeks).