iOS6 and Bluetooth OBDII Adapters

Dash ( http://drivewithdash.com/ ) is promising a Bluetooth 4.0 ODB-II module, but it's not available yet; it's a Kickstarter project as of this writing ( http://www.kickstarter.com/projects/shred444/drive-with-dash ).

Also take a look at Automatic ( http://www.automatic.com/ ), as Brad Larson mentioned in the comments above.

I am beta-testing Automatic right now. It isn't exactly a ODBII device, as much as it is a driving assistant. It does log ODBII codes, and allows you to clear the vehicle's computer when one comes up.

However, it also determines when you do hard brakes, hard acceleration, and when you exceed 70 MPH, effectively trying to coach you into better driving habits. It also logs your trips, gives you your gas mileage, gas cost, and when GPS is used, your complete trip is logged on a map, with time started and stopped.

the LELink device by http://www.outdoor-apps.com does work with iPhone.
I am also working on an application and
have so far issued an "ATZ" command and
got back "ELM v1.5"
using ios core bluetooth library.
You need a 4s or later iPhone.