Bought one of these kits today for £19 (not the shoes, just the iPod attatchment and wireless transmitter), they seem pretty nice and will definitly encourage me to go out running more (I already do around once a week).
Basically, the little sensor can tell you how long you've been running, how far you've ran, how many calories you've burned. It then uploads all the details onto iTunes and nikeplus.com for you to view and compare with previous workouts. Review here
The bad news? It doesn't work with my 30GB U2 iPod, or any other iPod, therefore I need to purchase a 1GB iPod Nano for an additional £100 and possibly the shoes for £65.
Awesome, but costly.