I voted "Completely Seperatable" I think that if they can operate independently that would be great. I do also think that it would be important for there to be powerful interfacing capabilities; like the example of making a new news item will start a thread in the forum.
Just off the top of my head it seems like it would almost be best if the portal and forums were both independent, and then there would be another set of code (module) designed for interfacing them. It would make installion more difficult, but I think it would encourage modularity. If it were done like this, it might also make it possible for others to write interface modules to integrate other scripts/software into the portal.
Any thoughts? I realize that this suggestion is kind of vague, but it could be developed