According to some developer forums, the Iphone refreshes apparently every 5 seconds. From reading around, it looks like the refresh rate is so bad, we can't even use the Hz measurement.
I have been testing the iphone app motionX GPS on my Wednesday night racing and comparing it to a 1Hz Garmin marine GPS. The knot meter fluctuates by 4 knots and our VMG and bearing aren't accurate either.
The average readings are accurate, but this isn't good enough to allow us to make tactical decision on a several mile long race course.
A refresh every 5 seconds might be perfect for highway driving, but this is going to be terrible for quick sequential turns.
Maybe developers can make the GPS readouts better by artificially simulating 1-10hz gps through the use of accelerometer data.