I've decided that once the ipod is calibrated at a certain pace it is measuring by the number of steps per mile. When you speed up beyond the calibrated pace, the number of steps/mile decreases and thus the ipod reads your distance short. The faster you run beyond calibration, the shorter the distance your ipod will read. Once you recalibrate you'll be fine. The problem is that most of us don't want to run at lactate threshold every time we run. One thing on my wishlist for a nike+ upgrade would be the ability to save calibrations for different kinds of runs. You could have one for your top pace, one for a hilly run, one for your average pace etc... That would solve a lot of these problems.
Anyway, sorry to hear about your wasted run. That's a smokin' time.
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