I have a demo MPiGuino sled running with the Adafruit LCD/button shield. Much nicer having only two wires (in addition to power and ground) to contend with. I changed the two zener diodes to 2.1v to avoid frying the 3.3v Pro Mini and will be testing it in-vehicle today. Because the Adafruit buttons do not generate a hardware interrupt there is no 'wake on button press' so I will verify wake on start.
T_Vago: Does ODO/Scratchpad add a known value to current or tank mileage? Asked another way, can I input mid-tank values from one MPGuino to another, so that I can switch Arduinos mid-tank, maintaining my current tank distance and tank fuel used?