If you plan to make the iphone your only gadget, then you may need the iphone sports armband from Belkin.
The ultra-comfortable Sport Armband is hand-washable and water-resistant. It gives you easy navigation access to your iPhone and minimizes bulk on your arm. This is the best looking one I have seen for use during your workout.
The flood of iphone accessories is probably only weeks away, so you may want to wait a little longer but for $29.99 this one is pretty good.