I really have no experience with I2C IMU's, but it would not be that hard to implement it, as you only have to change the code, that reads the g's (accelerometer) and rad/s (gyro), and then just use the complimentary filter or the kalman filter as I provided.
I presume that you know the I2C protocol? If not, have a look at the wire library: Wire - Arduino Reference
See the datasheet for the accelerometer page 14: http://www.sparkfun.com/datasheets/Sensors/Accelerometer/ADXL345.pdf and the gyros datasheet page 22: http://www.sparkfun.com/datasheets/Sensors/Gyro/PS-ITG-3200-00-01.4.pdf
Also check out the github for example code: GitHub - bitsmashed/6DOF-Digital: I2C interface breakout board for ADXL345 accelerometer and ITG3200 gyro .
- Lauszus