Jon Udell mentioned Biztalk's Internet Service Bus a while back. I'm just getting caught up with that.
Curiously, in this video they mention using WCF and Biztalk Services to implement a chat that can traverse firewalls, etc.
Have they considered not hurting the web and using IETF standards like XMPP? XMPP defines an HTTP-based client. And the Jingle (XEP-0166) proposal, em, which actually has some implementations, extends all *that* to accommodate higher-bandwidth, out-of-band, protocols for VOIP, includes firewall negotiation, etc. to figure out if they need to go through a server.
Ah, but, yeah, we could just use this Microsoft code and go non-standard all the way. Maybe I'm missing something. Or maybe you're happy in your little Microsoft corner of the internets.