Please review the Flash Player Security article

Does your Flex app rely on a crossdomain file anywhere? Are you using sockets? Do you call out to JavaScript? If any of these hold, please make sure to review the Player 9 Security Update article which explains some upcoming changes that you may need to take into consideration. We try not to break existing content, but in this case there may be some changes you’ll need to make, primarily in your crossdomain settings.

Read the article for more info.