For any runners out there, the 3GS is also compatible with the Nike+ fitness system. Assuming you have a pair of Nike+ trainers and the Nike+ sensor, you can fire up the Nike+ app on the 3GS, head out the door and start pounding the pavements. Including Nike+ support in the 3GS is a smart move for a number of reasons.
First up, if you're the sort of person who absolutely, positively has to be contactable at all times, it means you'll still be able to receive calls while you're out running and listening to your workout tunes. Second, you don't have to worry about plugging wireless receivers into a nano to make the system work, since the 3GS doesn't need a dongle. And third, anything that makes it easier to do a little exercise is a good thing.
Unfortunately I'm more of a cyclist than a runner, so I've never really been bitten by the Apple/Nike collaboration. Add to that, the fact that a proper fitness system with heart rate monitoring (like the Polar FT80) will give you far more accurate data to work with.
All that said, for many, the basic data provided by the Nike+ system will offer enough encouragement to keep pushing and get fitter. Also, there's a pretty large Nike+ community out there already, offering advice, support and those all important fitness challenges to keep you going.
Nike+ support will not be for everyone, but it is another example of the breadth of functionality that's available on the iPhone. And since your phone is likely to be the one gizmo that you have with you at all times, the more things it can do, the more useful it is.