It sounds like a great idea, and something that I would personally use. If that functionality doesn't already exist when they open up the SDK to third parties, I will see if I can make an app to do it myself.
The closest thing to that currently is the step counter. You can set the # of steps (in increments of 500) that you want to take in a day, so you can always see how close you are to that goal and walk around accordingly.