Sometimes it's hard to think with all the information.

Assuming you have BPM numbers input for your music (iTunes BPM input tool), it would be cool if the Nike+ in-shoe sensor could use your footfalls to set a tempo around which to select music on your iPod. Go running, listen to Coltrane’s “Giant Steps”, and feel the burn! A leisurely stroll would pull up something like Sonny Clark’s “Cool Struttin’”.
This is all the more reason that tracks downloaded from the iTunes Music Store should have BPM metadata embedded; once input by those responsible for initially tagging the music, individual users don’t have to worry about it, be they runners […]
Source: Daniel J. Wilson

Of interest, another popular story talks about how to get a free MP3 Player that may be an option for some people.