Sean from mp3Car reviews DashCommand for iPhone from Palmer Performance.
DashCommand stems from its window's counterpart that has been long since in car computers. It offers a wireless connection to your OBD2 port to bring you a ton of engine diagnostic tools.
With DashCommand for iPhone you can see all your available PID's in multiple skins, follow your acceleration and braking, see your g-forces going around turns, read your car's angle while driving, and keep all of this data for a log.
The only downside to DashCommand is that you can't swipe between screens, and that you need to tell it to switch to landscape or portrait mode before the sensor graphics look correct. That is because you will be able to customize the dash boards in later version.