Running while listening to music is so much easier if your pace is the same as the beat of your music. Someone should make a treadmill with an iPod dock that detects the beat of whatever you're listening to, makes it humanly possible to run to (perhaps by halving or doubling it) and then sets the speed to the beat.
Then, your playlist can become your program. Want a hardcore workout, put on some dance music. Feeling a bit tired, put on "Shine On You Crazy Diamond".
Image uploaded to Flickr by user 'Courosa'.