I've been really enjoying playing RFC MP via LAN, but unfortunately I can't seem to play as America. We've been playing as England and the Vikings and now America has just spawned but they don't show up as a playable civ on the choose-a-civ screen. Neither does England for some reason.

Any idea what might be the problem?

heh, just edit the CIV4CivilizationInfos.xml in the \Assets\XML\Civilizations and change their playability from 0 to 1. If these are "1" and still unplayable then... i would have no idea.