In what seems to be a waste of a potentially lucrative domain name, Stalk (http://www.stalk.com/) is a cell phone scavenger hunt. As far as I can tell, it was a game played on some college campus last spring.
The players are given photos from around campus with an indication of the general area in which each photo was taken. Their job is to run around and try to recreate the photos using their camera phones and send the images back to the home base.
It seems like a nice idea, although the project seems to be dormant at present. Which is a shame because they are sitting on a dictionaryword.com domain name. :P