Jordi spent the holiday well, laying out the physical ArduPilot Mega board. The last bit of this process is the routing of the traces, which will ultimately determine the size of the board. In the picture above, you can see the progression as the board gets smaller and smaller through each revision, finally ending up with the release candidate ("RC"), shown next to the current ArduPilot. It's surprisingly small, despite us using a much larger core processor with twice as many pins as the current Atmega328.
The main chips you can see in the board printot are the Atmega1280 (main processor), an Atmega168 for the PPM conversion/failsafe, and a multiplexer chip (failsafe).
You may not have realized it, but you've been watching much of the ArduPilot Mega development already:
The IMU, which is modular and will plug on to the top of this board, is functionally the same as the current ArduIMU+ and will use the same code. If you already have an ArduIMU, we will release an adapter board that will allow you to use it so you don't have to buy another one. But the custom ArduPilot Mega IMU will also have a differential pressure sensor (airspeed) and an onboard FTDI chip and USB port so ArduPilot Mega will have native USB support (hurray!). If you're not using the custom IMU board, you'll be able to program ArduPilot Mega through the usual FTDI cable, like regular ArduPilot.
The core ArduPilot Mega code will be built on the current ArduPilot 2.5, but with extensions for the new functionality that comes with 2-way communications, more memory and eight channel control. We'll also be adding more powerful mission scripting and event triggers, now that we have so many spare I/O pins and channels.
The PPM/failsafe circuit is based on our current 8ch PPM board.
The result of all this parallel development is that ArduPilot Mega will move pretty quickly now that we have a release candidate board, since so much of the tricky stuff like the IMU is already done, tested and mature. I'd predict a release in March or April.
FYI, the board is also build for expansion, and one of the four serial ports will be available to expansion boards, as will several I2C/SPI pins. So, for example, adding magnetometer, absolute pressure sensors (altitude), or ultrasonic sensors could be an option.