I've developed a fuel efficiency tracking app for iPhones, iPods and iPads that utilizes Wi-Fi OBD-II sensors. It's called OBDDrive and it's already in the AppStore.
Wonder if there's anyone who would like to test for free!
Check the website for screenshots:
It currently supports OBD-II compliant vehicles with a MAF sensor using gasoline. I'm planning on supporting cars with MAP sensors, etanol and diesel in the near future.
If you have an Wi-Fi ELM327 device and are interested in testing the app, let me know here in this thread. Just letting you know that I have to limit this to 10 users max for now.