We use Harry's Lap Timer, an iPhone app that integrates OBD and improved GPS data. Find it on the app store. Basic, Petrohead ($19) and GP ($27) versions. Go GP. Then order a Dual XGPS150 5hz bluetooth GPS ($100), and a GoPoint BT1 OBD bluetooth device ($100), all from Amazon or company store and you're set. All integrate and communicate with Lap Timer. His home page explains.
There are also desktop and iPhone apps for the OBD unit that let you diagnose and reset engine codes,, bla, bla, bla.... important and useful though.
We're still waiting on a brake pressure sensor add on to Harry's , he told me today he was working on it. Now that would be nice. His app currently has great acceleration curve data in the lap data section. Go for it.
Oh, you'll also need a suction cup mount for your iPhone, it goes just under the rearview mirror. That's how the whole thing works. Punch it on when you pull out and it starts automatically or on command, gives you great video - it's part of the Harry's Lap Timer function.
