I'm using Andoid, so I don't think the app is available for iOS. But since I know it will come up, I figured I'd post anyway. The OBDII adapter will work with iOS anyway.
Check out the Torque app. It does everything you could possibly need it to do, and just about everything you would ever want. I seriously cannot say enough good things about this app.
I use the ELM327 (can be found on eBay) Bluetooth OBDII adapter. Their reputation is supposedly hit or miss, but mine works perfectly.
This setup cost me a grand total of under $20.