When I first joined the gym I took a few sessions with a personal trainer. One of the things that he ingrained in me is to never read or watch TV while working out because doing so causes the intensity of a workout to drop. Instead it's best to listen to an audiobook or music.
A while back I read a New York Times article "Phys Ed: Does Music Make You Exercise Harder?" explaining why it is important to listen to music when working out. The reason lies in our nature of wanting to move to the beat, something we can't always control. There is also the psychological component, where music allows us to divert our focus from the seconds hand on the clock, pain, and exhaustion.
Essentially, it can make your workout more productive AND more enjoyable.
Here's what I'm listening to while puffing away on the elliptical:
What are you listening to when working out?