Last summer a friend and I shot this footage, but I just got around to editing and posting. It was inspired by the notion of "Schelling points". You can see a brief Wikipedia article on them here.
One of the two teams was made up of friends of ours, and the other responded to a CraigsList post. We did not tell them anything about the task or the rules beforehand and offered no prize. We did, however, tell them we would buy them dinner and drinks afterwards regardless. The two teams had never seen or heard of each other.
We had no idea what would happen.
Possible future variations:
Not giving them t-shirts
Giving them a photo of the other team, but taking it away before they start
Giving them a photo of the other team, and letting them keep it
Giving them only the names of the other team
Making the playing area both Boston and Cambridge, or even some other larger city
Making it so there are three teams, and the only the first two to find each other win
Leaving it only two teams, but having some other person or team, who has to try to prevent them from meeting