• Modified ArduPilot2.6 and ArduIMU to do interprocessor communications via TWI to free up USART for full-duplex serial comm's between ArduPilot and GCS; can upload PID coefficients in flight. IPC code example: twi_stuff.zip
• Test flown in manual and stabilize modes, IMU appears to be consistently and accurately reporting attitude.
• Stabilize mode is solid with no oscillations using default PID coeff's, but need a way to trim pitch; taildragger isn't at cruise pitch attitude when IMU is initialized.
• Using SUP500 GPS module from SparkFun; altitude values off by up to 100 meters (field elevation 1800m). GPS frequently reports altitude of 1700m at pattern height (0-20m AGL). Also seeing huge intermittent (1 second duration) errors in lat,lon on the order of +/- 2 degrees. I don't think I screwed up the NMEA parser...
• Also switched to a Serial library which performs buffered transmits. This seemed to solve some problems caused by the standard library's blocking writes. HardwareSerial.zip

For those with the 16HMz AT328P version of the SparkFun Razor9DOF board who also want to run the DCM based AHRS code, this may be of interest.