OBDii for iPhone...

[technocoy]

This looks cool too...

http://www.devtoaster.com/products/rev/

Here are the other options taylor listed:

http://itunes.apple.com/us/app/dashc...321293183?mt=8

http://itunes.apple.com/us/app/from_...27822395?mt=8#

Both look like good alternatives as well.

This one is free, but the interface suffers a bit of course:

http://itunes.apple.com/us/app/logwo...351896392?mt=8

FuzzyCar is free as well and also has an app they make that is only for code pulling if that's all you need:

http://itunes.apple.com/us/app/fuzzy...319004364?mt=8