Wouldn't a separate login system for the portal be better? Since you might get people who just want to keep up-to-date with the news and articles available in the portal system only.

Though i can see the other way having its positives as well. It would be harder though. If you wanted them intertwined (forum and portal login systems) then i am not sure how JM feel about it but would they consider using the vBadvanced or what ever the portal system is for vBulletin and just customizing it to there liking?

<edit> just noticed my post brings up almost the same identical ideas as Rob's Sorry