I'm old enough to remember the Walkman Revolution. It seemed like all of a sudden you could jog around the neighborhood listening to whatever you wanted, all courtesy a cassette deck and a pair of small headphones. Actually, the current iPod Revolution looks, well, pretty much the same.
Except that the iPod holds several hundred (or is it thousand?) cassettes-worth of music, and it's a lot smaller, and it runs a lot longer before it needs a recharge. Still, the same problem emerges: What if you want to share your music?