I have had good results at an autocross event with the i-Blue 737 external GPS. The update rate this past Sunday varied between 5 - 10 Hz and the reported accuracy was 2 ft (61 cm). This was my third event with good consistency. For this event the start line was back about 15 ft (5 m) from the start lights. I never got a pre-mature start and attribute this to the good accuracy.
This GPS requires a Jailbroken iPhone and the BStack GPS app to allow use.
Thanks Rob for the feedback. Please note that accuracy reported with BTStack GPS is not valid currently, it is just a default. Not getting laps started by accident so near to the trigger is a good indicator for good accuracy, that's true.