You gotta love the guys at Sparkfun. We had a small compatibility issue with the GPS daughterboard
that was caused by the polarity protection diode on ArduPilot that has the effect of dropping the 5v Rx voltage down to 4.3v on ArduPilot. Not a problem for ArduPilot, but could be a problem with voltage-sensitive add-ons. I asked Nathan at Sparkfun for advice. Not only did he solve the problem (the next production run of the boards will have a power regulator), but he wrote a whole tutorial
on the issue!