Can't wait! Live multiplayer would be a lot of fun! Be sure to set up an official server.
(Also note that griefers would first have to find whatever it is that you made, which will not be very easy...)
What do you think the limit of players/server will be around with the engine's current state? I think right now the server load would be somewhat light since currently you can pretty much only just drive/fly around, place static objects, and make craters. Also there would be a good chance that at any given time, not all (but still maybe the majority) of the people on the server would not be close enough together that they would need to be sent updated information about each other. Heck, if UFO mode still used no model, you would only need position updates for vehicles and crater projectiles, at least I think anyways.
I don't really have any experience yet in working with networked games, so I could be missing something that is significant networking wise.