I take it back. On the original SharkFin, the analog circuitry was causing a small error in both the gyro and accel measurements. We've since corrected the design, but we didn't want to lay money down on a new set of boards until the concept was proven.
Along comes the 9 degree of freedom Razor IMU from SparkFun. This $125 bad boy has 3 axes of accel, 3 of gyro and 3 magnetometer, overkill for sure. I got this for experimentation and found out that it fit beautifully on Fin. The holes even lined up. This amazing foresight by Anool, enabled the sensor swap.
The 9 dof razor has a ATMEGA328 on board already. I reprogrammed the MEGA on the SharkFin base to be a digital slave dummy. It just listens on the serial port for 3 byte messages which it assigns to the digital ports: PORTB, PORTC, and PORTD. This simple interface could be expanded to also read digital inputs from the slave.
The accel datasheet indicates a selectable bandwidth. It is unclear on exactly how this is being accomplished, but if it is effective, this will relieve the huge burden on the processor of low-pass filtering the accel data.
The magnetometer offers the possibility of complete head orientation tracking. It is still unclear exactly how to take advantage of this given that there is no true velocity measurement. Ideas are welcome!