I have been trying to play multiplayer for the last couple weeks, but always get the "Unable to connect to server" message. I've tried from multiple internet connections, and have never had it say anything else.

Is multiplayer dead with the game, down for a prolonged problem, or something else? Are there any requirements in terms of networking/firewall beyond the generic allow outbound connections and established / related inbound? Is there something I need to configure before I'm allowed to connect?