There's another app out there, Dynolicious that just uses the accelerometer to estimate hp, 0-60 and other stuff. This is different. The free version offers up a G meter based on the accelerometer and a track tool thing that uses the GPS to show accel/decel on chart based on location. Its pretty weird to see but its supposed to be useful to driving around a track, showing you how you take the corners or something.
The OBDII reader part isn't out yet though.