If any early adopter dumps all their coins on the market at once, the value of your bitcoins it at risk. That's about the worst they can do, unless they're evil geniuses of some kind that come up the kind of brilliant scheme that only works in movies.

An example of such a 'movie plot' attack would be to abduct the people who control the forums and the release of the client, create a fake urgent bug report using fake forum posts from many well-known forum users, and induce most of the network to upgrade to the latest version of the client. Somehow, you'd have to stop people from noticing and finding other ways to get out the word. And then use the tampered client to pay for everything you possibly could in fake coins, cleaning out all the exchanges. (In case it isn't obvious, I don't think such an attack could ever pay off enough to cover the costs of launching it.)