Seth Godin has a blog about the end of digg/reddit. He creates a cute analogy to the "pencil drop", where the students in a class will all agree to drop their pencil at the same time to annoy the teacher. He argues that this sort of effect will lead to shilling on social news sites, where a group of people will all agree to take action at the same time to get a chosen article seen.
One possible counter to this phenomenon is the personalized social news site, where each user sees the news he or she wishes. On this site the shill has less to gain, because he can influence only those people who agree with his past decisions, and shills will have limited effects, because people's profiles will learn to ignore them. (On the other hand, personalized shills may still be a problem, but that's another story.)