Unencrypted beacons can be hijacked by another app... The iBeacon protocol does not provide encryption. #iBeaconWebinar
We didn't go into great detail on the webinar, other than a statement that the protocol does not support cryptographic integrity. Anybody is free to copy your numerical identifier (UUID+major+minor), so it may be important to cross-check a proximity location against other data before proceeding with a "trusted" transaction.
Basically, you cannot do anything security sensitive based on proximity and can only use the information in an app as a guide or convenience feature to improve the user experience. Traversal of an actual security boundary must entirely rely on other means.