Wouldn't it just be easier to setup private forums with moderators and let them allow/deny access to a general forum as they see fit, alliance based or otherwise? I think a "permanent" alliance runs into a lot of issues where alliances are auto-created in game worlds with zero members or the alliance admins aren't correct etc. While "permanent" alliances are defacto, I think most players should be encouraged to not always be apart of the same alliance every gameworld because you end up creating an "us versus them" environment where people end up holding grudges, calling people cheaters, etc. Certain players have been labelled without permanent alliances in effect and end up guilty by association.
What if a player is part of a permanent alliance and wants to join a different alliance for one game world? That means the player is potentially seeing information from "the competition". There are a whole lot of issues with a permanent alliance IMO, but having non-gameworld specific groups would be useful so that if players do want to organize into a "permanent alliance' they can, or if there is a group of players who like to watch American Idol they can form an American Idol group, etc. The idea being that they aren't permanently committed and/or associated with an alliance, but a sub-community of other players, whatever that ends up meaning.