@CarlOlsson, did you use the Arduino Wire library for the Arduino side of your project? I've got my driver to compile on the Pixhawk, but it doesn't seem to recognize the Trinket board. The Trinket (3.3V version) is set up as a slave sender using this version of the Wire library. It communicates fine with another Arduino, but not with the Pixhawk.
I'm also using and I2C-based airspeed sensor with this Pixhawk, and to get it to work I had to download Mission Planner and set parameter ARSPD_PIN to 65. Is there a similar procedure to make the Pixhawk recognize other custom sensors? (Not exactly related, but where/how is the ARSPD_PIN parameter used? As far as I can see it exists nowhere in the PX4 codebase...)
Edit: Figured out part of my problem, I needed to add the start command for the driver to ROMFS/px4fmu_common/init.d/rc.sensors under the right "if" statement for my board. Now on to the next quandary--why is it always reporting 0 rpm...
Edit again: I was sending one byte from the Trinket and the Pixhawk code was still trying to read two. Works like a charm now!