I've found (thus far) that didDetermineState: ... is the only ESTBeaconManagerDelegate methods that's signature is ESTBeaconRegion but in fact returns a CLBeaconRegion object.
I maintain a dictionary keyed on ESTBeaconRegion which value equates to the ESTBeacon for the given region. Getting back a CLBeaconRegion causes my dictionary query to fail on objectForKey:. I can work around this by converting the incoming CLBeaconRegion to a ESTBeaconRegion, but thought you all would appreciate knowing in order to fix for future releases.