Small Arms-Viral Achievements

Xbox 360 Achievements are hot these days. Everybody loves them. Small Arms, developed by Gastronaught Games, has achievements of its own. One of them, called "six degrees of small arms" is viral. It works like this. Four developers at Gastronaught started out with the achievement, every person they play automatically gains the achievement as well. Every person they play gets it too, and so on and so forth. I think this is a very cool system that I'm sure has been a ton of fun for players. Its also very different from the norm. Its a creative approach that offers further incentive and reward for online play. I wonder how many people have the achievement by now?
If you were to create a 360 game, what type of Achievements would you give? More broadly, what kinds of fun creative incentives tat fit your game can be offered to players? I just wanted to let you guys know, wanted to get you thinking about possibilites.

Peace.