In April 2008, Adobe imposed new security restrictions on socket connections made by Flash Player. Starting with Flash Player 9.0.124.0, all socket connections made to a server require authorization via a socket policy file.
For a general introduction to the issue of mandatory socket policy files, see Unity technote 00005.
For full, gory coverage of the security changes made in Flash Player 9.0.124.0, including their motivation and required action, see the following resources:
>> Setting up a socket policy file server
>> Understanding Flash Player 9 April 2008 Security Update compatibility
>> Policy file changes in Flash Player 9
Unity users can meet the new socket-policy-file requirement by following the instructions in Unity Technote 00001.