OK this is one I\'ve been trying to figure out for a few days now. We have an application on Windows Phone 7 where phones join a multicast group and then send and receive me
Have you ever tried joining another multicast group? Because 224.0.1.1 seems to be in use as of IANA assignments. You find all of the here.
Maybe on Windows Phone 8 some service is tighter bound to listen to messages coming in (e.g. a network service that is running in kernel mode) and they are never forwarded to you.