If you send headers to a domain that does not match the domain where the SWF is hosted, you need update your crossdomain.xml file to allow them to continue to work with Flash Player 9.0.124.0. While this is discussed in the ADC pre-announcement, it could use a bit more detail. Flash Player support wrote a great technote on details of how to set up <allow-http-request-headers-from>.

http://kb.adobe.com/selfservice/viewContent.do?externalId=kb403185

One of the examples from the article is:

<?xml version=”1.0″?>
<!DOCTYPE cross-domain-policy SYSTEM “http://www.adobe.com/xml/dtds/cross-domain-policy.dtd”>
<cross-domain-policy>

    <!– This domain can accept a MyHeader header from a SWF file on www.example.com –>
    <allow-http-request-headers-from domain=”www.example.com” headers=”MyHeader”/>

</cross-domain-policy>

Also, it is worth reiterating that the new header crossdomain syntax is required for both send and sendAndLoad network APIs. For network requests without additional headers, traditional crossdomain syntax and behavior continues.