Harrison Hoffman has an interesting article up on CNet about the Wisdom of Crowds failing. The phenomena that he's really observing though is a gaming of the system at IMBD. People are voting the Dark Knight movie up and the Godfather down in order to influence the rankings.
Explicit information where people take an action to rate or vote can be wonderful and create fantastic resources. However, it doesn't matter if you're at IMBD or Digg or Yelp it can also be gamed.
At Aggregate Knowledge we have found that Implicit information where systems look at what people are naturally doing tends to be much more reliable.
It's not what you say - it's what you do!