Is something like geofencing with beacons possible with the new Google Proximity Beacon API? Is there a callback when the user is entering or exiting a zone around a beacon? I couldn´t figure it out in the Google documentation.
The Proximity Beacon API is best thought of a database mapping beacons to attachments and interesting registration data, so it doesn't really provide much functionality on the client in terms of fencing and zones.
You have the ability to work with and/or write some Beacon Scanners which will tell you when it's first sighted a beacon and subsequently lost contact with said beacon. There is a bunch of sample code for this in the Beacon Platform Sample Apps.