Now that I can use my iPhone as a heart-rate monitor, I wish someone would invent an app that I could use to set up my workouts by desired target heart rate, including such variables as the length of time I want to get to the peak heart rate during the exercise period, how long I want to stay there, and how long the overall workout should be. The app would then select iTunes from my built-in iPod that have the correct beat to correspond to my desired heart rate (starting heart rate building to target heart rate, and then decreasing again at the end of the workout). Then I could run in time to the music and actually listen to the music and work with it instead of thinking about my heart rate and how much longer or faster I have to go.
So get on it, and let me know when it's done. I'll even pay. A bit.