Measuring walking speed with iPhone accelerometer
I don't have any idea about Accelerometer in iPhone, but converting Accelerometer data into distance is fairly difficult because the offset in double integration causes drift as already told. Check this interesting presentation sensor fusion for further details.
As far as I  know this can be avoided by using Digital Accelerometers( with high sampling rates), double integration has to be performed using electronic integrators with appropriate filters at every stage to remove the offset as well as the low frequency noise.
Also in that presentation he mentions the possibility of using more sophisticated algorithms like use of Kalman Filter for object
tracking or Pedometer Algorithm for pedometer design.  

Hope this gives little more insight into your problem:)  

PS: This does not accurately address your question still I thought I will share my knowledge, I want to comment but I don't have enough reputation !