A well interesting article on as to use Flex 2 with Flash Media Server, is a simple example but of the one notion of as to use the class NetConnection and NetStream in Flex 2 and to feel some differences in the Action Script 3.0.
link of the article: http://flash-communications.net/technotes/fms2/flex2FMS/index.html
See the application working
14/03/2006 *** UPDATE ******
I found another article on Flex and Flash Media Server written by Christophe Coenraets, very good for signal!
Link of the article: http://coenraets.com/viewarticle.jsp?articleId=98
Link for the application