The only hitch is that you need an OBD-II/WLAN interface that can beam the information from the port to the phone. DevToaster is working on that now; in the meantime, you can download Rev Lite at their site and we have a feeling California's going to work on more phone-centric activities to ban
via WRX Fanatics
Here's another iPhone application that demonstrates that soon there is nothing you won't be able to know if you have just one device. DevToaster has created an app called Rev that reads information straight from the OBD-II port on your car and displays it on your iPhone. When it's fully operational, Rev can display things like vehicle speed, RPM, fuel consumption, calculated engine load, and a host of other parameters. You'll even be able to check and reset engine and error codes.