I use MotionX GPS on the iPad, mostly for off road situations. There is an iPhone version, but I've never used it. It looks about the same, and if so, I'd recommend it.
I wrote a mini review in the thread "On the road and off it too" on iPadForums. If you need any more info about it give me a shout, either here or in the thread on iPads. I monitor both forums.
Edit: it's accurate to 6 decimal places, which is about 4 feet at the equator. Not exact, but pretty close.