How precisely can I track distances using the iPhone accelerometer?
From what I've heard, the iPhone accelerometer output is very noisy - most applications that utilize it for any sort of moderate precision work do a lot of filtering and output modification to smooth out the noise.  The iPhone can be placed flat on a table, and the accelerometer will still output a fair bit of data.  I imagine you could certainly use it to measure longer distances (>10m), but I probably wouldn't trust it for high resolution measurements.