In the NY Times, John Tierney reports on a study of the impact of gossip on judgments. The bottomline, when it comes down to it, we believe gossip despite the evidence of our eyes. Or, more accurately, gossip sways our decisions even though we know the full story. (Of course, the study was a bit unrealistic--we rarely are in situations where we know we know the full story.) The only positive bit--positive gossip had as much power as negative gossip.
Maybe this explains the recent reports on the Obama is a Muslim urban legend, or the anti-McCain gossip in South Carolina in 2000--the gossip persists and spreads because we're tuned into it as intrinsically valuable, not only do some of us believe it, but it's worth passing on.