Ok so from all my research I'm fairly positive P2P is not possible in Silverlight 4 (in browser).
Flash can do P2P using Stratus/RTMFP which I believe uses UDP hole punching and a Stratus server to help initialize the connection.
Is there any plans for Microsoft to add some kind of p2p rendezvous service in the future? Can we get a definitive yes/no on whether or not p2p is possible in SL4.