Adobe has announced plans to release a security update that could significantly impact existing Flash applications that utilize cross-domain communication.
To quote from their post:
If any of the following situations apply, you should read this article in detail:
To defend against malicious HTTP headers, the update requires a cross-domain policy check before allowing SWFs to send headers to another domain.
If you perform any cross-domain communication in your Flash applications, especially those compiled for Flash Player 7 or earlier, you should really take some time to read up on this release.