I recently purchased and installed the optical flow sensor on my quad.  During initial tests in CLI mode the optical flow sensor could not be initialized.  After looking at the datasheet for the sensor I noticed that the NPD line needed to be held high for the sensor to communicate with the APM.  The instruction in the wiki for this sensor do not show connecting the NPD line.  I connected a jumper wire from the NPD pin to the 3V pin to hold this line high all the time.  This fix allowed the sensor to initialize and start communicating with the APM.  Has anyone else had the initialization error I saw and either used this fix or found another way to fix this?