Okay, I learned one today that I didn't know, and probably is known by everyone else but me.
Now, I did know that the iPhone headphones have a microphone - it's that little white plastic bit on the wire leading to the right earbud. What I didn't know was that that microphone is also a control device. Squeeze it. Go ahead. It clicks, right? So, that can be used to start and stop music, skip to the next track (double click), and move back to the previous track (triple-click).
Most of the time, my hand is actually closer to the phone than to the headphones, but it's still very cool to know about this.
I picked this bit of info up from the always excellent John Gruber over at Daring Fireball.