The federal Centers for Disease Control and Prevention recently issued a flier to combat myths about the flu vaccine. It recited various commonly held views and labeled them either "true" or "false." Among those identified as false were statements such as "The side effects are worse than the flu" and "Only older people need flu vaccine."
When University of Michigan social psychologist Norbert Schwarz had volunteers read the CDC flier, however, he found that within 30 minutes, older people misremembered 28 percent of the false statements as true. Three days later, they remembered 40 percent of the myths as factual.
Younger people did better at first, but three days later they made as many errors as older people did after 30 minutes. Most troubling was that people of all ages now felt that the source of their false beliefs was the respected CDC.
the fact that people remember the issue and forget if say the CDC says it was true or false means that it is always easy to confuse people with a controversy (say about John Kerry's military service). It makes it easy for someone who can get the right words into newspapers to change an issue from one where people disagree with you to one where they are confused.
I think the observation is not exactly new. IIRC it is actually one of the older results in psychology having been demonstrated by Edward Thorndike in an experiment in his lecture hall.
update: Thanks for links here and here.