Need to do some better lighted pics for the final project writeup, but here's my finished Untappd enhanced kegerator.
How it works:
An interrupt routine on the Arduino calculates the RPM of flow meters installed in the tap beer lines. Once it notices a tap flowing, it performs a beer "checkin" API call to api.untappd.com, lights the corresponding LED, and begins a 5 minute "lockout" timer to prevent multiple check-ins (from topping off a pint, etc). If the connection to api.untappd.com fails for some reason, the LEDs will flash alternately to signal the fault and the lockout will be cleared. For kegerator maintenance (or pouring a pint for the neighbor), a button can be pressed to also suppress check-ins for 5 minutes. Both LEDs light in this case to signify the bypass. The Arduino also serves out a simple web server to the LAN for conveniently changing the Untappd beer IDs currently on each tap. Updates are saved to EEPROM.
There's lots of other cool Untappd API calls. I'd really like to have a small display (maybe Chumby sized?) on top of the kegerator, showing my Untappd friends beer check-ins in real-time. Also got an eye for Adafruit's PN532 RFID shield. It would be cool to hand out RFID tags to friends so the kegerator could recognize and check-in their pulls too! Some sound effects for Untappd badge awards would also be fun (and easy) to code.
Here's the Arduino code, if you happen to feel like building your own untappd kegerator: http://pastebin.com/i2nBqicg Pretty much all the parts, including the flow meters, are available from Adafruit Industries.