A seriously-techie colleague recommends Rev Lite from DevToaster as an IOS OBDII app as the one he uses. I have it downloaded and like the format, but I haven't bought a Bluetooth scanner to feed it data yet. They're kinda pricey. I'll edit this post later when I have my notes on his scanner recommendation in front of me.
Not sure about IPhone, but I use this and an App called "Torque" on my Droid. The bluetooth OBDII scanner should be the same, you just need an Iphone app to interface with it: