UPDATE: BLE112 firmware source code can be downloaded here: BLE112 iBeacon firmware. It should be pretty well documented so please try to figure it out yourself before asking :)
Here's a snap of the first two "production" iBeacons (not really production models but a lot prettier than the previous mess-o-wires prototype). They are based on BLE112 Bluetooth modules and contains nothing more than a coin cell battery, a tiny DIP switch for configuring the device identifier without re-flashing the firmware, a capacitor and a diode for reverse polarity protection when external power is used instead of the coin cell battery. And a BLE112 module, of course.
Here are the Eagle files for schematic and board:
The firmware source code can be downloaded here: BLE112 iBeacon firmware.