The wolrd is lucky !
It exists a very robust and interoperable middleware : XmlBlaster at http://xmlBlaster.org.

To have a fast overview of that fabulous and incredible peace of success, have a look at http://xmlblaster.net/.

Here it is a PDF doc of all xmlBlaster's requirements

http://giquello.com/xmlBlaster_requirements.pdf

original HTML page : http://www.xmlblaster.org/xmlBlaster/doc/requirements/requirement.html

I did a little demo to access xmlBlaster with Macomedia Flash using xml-rpc.
Read more about it in README.

Here it is an archive of this demo (with .swf .fla and xml-rpc stuff) : flash2xmlblaster_demo.20051110.zip(~400ko).

I wont ofen update this wiki page, so to be shure to get the las version or to know if there is new stuff, always check the xmlBlaster site at http://www.xmlblaster.org/svn/trunk/xmlBlaster/demo/flash or http://xmlblaster.net/FeatureMatrix.html.

here it is a demo. You should have a look at
http://xmlblaster.org/xmlBlaster/doc/requirements/client.csharp.html

here is the source code : xmlblaster_csharp_xmlrpc_demo_2005-11-18.zip

Even if you don't start up a client side callback server you need to configure it in the ConnectQos so that the server establishes a callback queue. By setting retries='-1' the server never gives up to reach the client.

Here is a Qos example :

<qos>
<session name="username/3" timeout="3600000" maxSessions="10" clearSessions="false" />
<queue relating="connection">
   <address type="XMLRPC" pingInterval="5000" retries="-1" delay="1000" />
</queue>
<queue relating="callback" maxEntries="5" maxEntriesCache="2">
   <callback type="XMLRPC" pingInterval="5000" retries="-1" dispatcherActive="false" delay="1000" />
</queue>
</qos>