Cracked a Multicasting problem this evening for one of my Canadian Tweeple.
They wanted to mulitcast some MSMQ messages to another machine but nothing was arriving in the listening queue.
A local queue could be configured to listen to the particular IP address/port in use and messages would arrive, though.
Looking at the network traffic, nothing was going onto the wire for the IP address/port pair until they looked at traffic to the VMWare adapter.
The machine had a virtual machine to simulate a remote computer and when they changed the setup from NAT to Bridge, multicasting burst into life.