Taken From the website:

Introducing: Dynolicious

Dynolicious is the first automotive performance meter available for the iPhone and iPod Touch. Dynolicious uses the hardware built in to your iPhone to measure the performance characteristics of your vehicle. No external wires or equipment is necessary, simply slip the iPhone into a cradle of cup holder and go!

Some of the performance metrics in which Dynolicious can measure:

0-60 acceleration
1/4 mile elapsed time
Lateral G's
Horsepower
Unlike other accelerometer-based performance meters, Dynolicious uses the advanced data-handling and display capabilities of the iPhone to make the most of your performance testing. Dynolicious maintains a history of test runs, showing your averages and trends in your results. Dynolicious also allows you to enter modifications performed to your vehicle, and will instantly show before-and-after results to easily identify gains or losses.

Here is the website - http://www.dynolicious.com/

And some chat about the product on another forum - http://www.e90post.com/forums/showthread.php?t=159715

For £7.00 odd im gonna get it, try and get a hold of it tonight and get out for a 'test'