Quote from TriPack:
Rumors and news of this type do have a market impact - but the impact is typically to exacerbate the underlying trend. But some rumors are taken off the shelf and used over and over to push the market this way or that. The "Saddam", now "Bin Laden Capture" rumor is one example of this. I can't recall how many times Bin Laden has already been captured by false news reports used to move the market.
I think this is what Cheese and sulong are talking about - because these news stories tend to revert back to the mean - like today's movement, for example.
News that is of a truly "surprise" nature and that is significant to the market fundamentals (like a surprise rate move) will change the trend direction for more than just a few minutes or hours, but these news reports are extremely rare.