Because the iPod stores music digitally, I don't see how the hardware could change the speed at which the music was played. A record player (anyone remember those?!) or a tape deck can run fast or slow, and thus make the tempo or sound of the song change, but I don't think a flash-drive iPod could do the same.
I wonder if it's just psychological. Kind of like how some music (speed metal, for instance) sound especially "fast" when you're driving down the highway in a convertible on a warm summer night. (Yeah, I was a teenager in the 1980s!)
Anyway, maybe the song you're listening to is simply already a slow song, and seems even slower when you're feet are moving at a much higher "BPM" than the music itself. Your feet are telling your brain the music is too slow, and your brain is conveying that message to your ears.