This is a great idea, and actually a "must-have" feature for me. I have a board with over 80 different fourms, but most of my users might only be interested in viewing/posting in a dozen or so of them. Cal did a hack for this feature for the UBB, and creating one like it for vBulletin I would imagine wouldn't be difficult. For an example of what this looks like, take a look at
http://www.progresstalk.com/
-Chris