The 3G doesn't have the proper HARDWARE to recieve the signals from the Nike+ shoe insert. It's nothing to do with software.
The Nike+iPod kit comes with 2 pieces: a transmitter (for the shoe) and a receiver that plugs into the iPod nano's dock connector. I think what people would like to do is plug that receiver into their iPhone 3G and use the system the same way it is used with the iPod nano.