The iPhone has a pretty nice feature for fitness fans. You can purchase a Nike+ pedometer which you attach to your running shoe (or even buy cool Nike ones with a little slot ready made for it). The pedometer communicates with your iPhone when you’re running to tell it how far you’ve gone and you can setup music playlists which encourage you along the way. When you’ve halfway or near the end of your run the iPhone will give you verbal encouragement over the music. Nice. Once you’re done you can upload your data online and challenge others around the world to races etc.
How about taking it to the next level, using the augmented reality capabilities of the iPhone by running against other people LIVE. So when you lift up your iPhone you can SEE your running partner behind or ahead of you, even though they may be elsewhere in the world. Using the audio connection capabilities of the iPhone you could talk live to your partner and make that audio spatial – so as they run further ahead or behind you, you hear them getting quieter.
Take it a stage further and you could have a virtual fun run or marathon where all the participants are in different locations.
People in gyms could run against those running outside – one using the Nike+ pedometer function and one using GPS detection to measure speed. By plugging your iPhone into the gym screen on the treadmill you could see permenantly when someone is ahead of you, or show a rear-view mirror type of view if people are behind you.