Suggest you post on the mod_cluster user forum at http://www.jboss.org/index.html?module=bb&op=viewforum&f=288 where more people may see it and help.
I'm going out in a minute so have no time for a full reply, but will say this:
That message is coming from JGroups, not from mod_cluster itself. One *possibility* is one of the JBoss AS clustering channels is inadvertently picking up the "advertise" multicasts that httpd can send out if the "advertise" feature is enabled. If so, that becomes a configuration issue to separate the traffic.