Most of the discussions related to XIFF ( XMPP AS3 library) are about its lack of examples and documentation. Lately the whole project has seemed to be forgotten by its original developers. Perhaps it is just due to the lack of time.
According to the many new beginners with XIFF, the best reference application made with it, is the XIFF GUI by Nick Velloff in the late 2007 (v1.3).
Here is yet another update (v2.1), in order to make it be useful with the current state of the XIFF project.
Once you have the application running, there comes another kind of obstacles. The two most popular XMPP servers seems to be eJabberd and Openfire, of which only the latter provides the cross domain policy file correctly. eJabberddoes have a patch to add that functionality.