Apparently - Nike and Polar are going to be releasing a heart rate monitor that can be integrated with the Nike+ system.
Seems like a good fit - you can measure your HR + pace/distance/time.
If you're doing most of your workouts solo (like myself) I'm guessing you could follow a pretty comprehensive training schedule?
I know people on here are too cool for these things and will say to just go out and run your workouts/long runs. But it can be a good way to prevent over-training but keep your intensity up for hard workouts.
Any thoughts?