There needs to be an App for the iPhone where you tell it where you need to walk to and stick it in your pocket.
It vibrates one way, turn left, another turn right. So you walk down the street and seem to know where you are going. No more looking down to see where you are instead of looking at the nice people on the street.
Also, you could have one that sends a tone to your headset..or even better, says "Hot" or "Cold" as per the directions of that game.
You could have a game where once every minute you are told if you are hotter of colder to some one else, and every 90 seconds the person you are looking for is notified that that are hotter and colder.
Two people could find each other easily, or turn it into a hunter/hider game.
Perhaps some sort of real person city wide Chess variant could be created?
Remember the game Assassin? where you got a contract and tried to shoot someone with a dart gun? same deal, except you try to get within 10 feet of another player and the iPhone communicates a kill...make the kill based on a random roll, modified based on some parameters.
Of course, this could be done with any Smart phone, but I am hoping mentioning the iPhone will generate hits.
If anyone is thinking that I should write the App, they are correct. it is well within my software skill. Just send me an email and I will tell you where to mail the iPhone to...