Head on over to the iTunes section of Apples website and you will find full details of the small receiver that plugs into the dock connector of your iPod. You get a free Nike.com account, custom workouts and when you sync your iPod and your workout data synchronises with iTunes and your Nike account.
Seems also that the site only refers to the Nano, though I think 5G iPod compatibility will be a must by the time this launches.
iTunes delivers you specially selected mixes to workout too, Nike provide (or should I say sell you) the special Nike+ trainers. Available late June.
Wicked or what.