I have completed ground testing on the new firmware I have written for ArduPilot to use ArduIMU in place of the thermopiles. It has been snowing here and I probably won't get to fly the code for a bit, but figured others might be interested in giving it a try at this point. This firmware will probably exist on a temporary basis, with the code getting incorporated into the regular ArduPilot firmware in the future. If there is sufficient interest in this approach I will consider working on a ArduPilot shield board that would add the pressure sensors to ArduIMU and package it all as a shield.
I will maintain the code for a while in a repository at http://code.google.com/p/ardu-imu-pilot/
If you try it out give me your feedback. It is similar to ArduPilot 2.4 but uses a different header file.
To interface ArduIMU to ArduPilot you connect the gps (ublox only) to ArduIMU, provide common power and ground to ArduPilot and ArduIMU, and connect the serial output pin of ArduIMU to the serial input pin of ArduPilot.