I ran across this post at MindHacks about how retractions of falsehoods actually reinforce the belief in them. People tend to retain the item as truth the more it is repeated, rather than discounting it when they hear that it’s false.

Also

The researchers note that people are more likely to discount information if they are suspicious of the motives behind its dissemination.

This would suggest that it’s very difficult to reach someone with your message if they are suspect of why you are delivering it. How does that relate to the infowar in the blogosphere and the Civilian Irregulars who are attempting to flood the message boards with the Truth of what’s happening in Iraq?

if you want your propaganda to be effective get it in early, repeat it, give people reasons to be believe it (however irrelevant), and make yourself seem trustworthy.