I like that...
You could do it with Javascript and just keep the unselected tabs hidden from view.
That is slick but it's just DHTML menu. You could do that with code from dynamicdrive.com and lots of template changes to mybb.
Didn't really like it, plain gimmick which gets boring after a while. I like to have a listing of forums without requiring me to click again and again.
.. and as labrocca mentioned, it can be done using simple DHTML (which is a usage of HTML, CSS, DOM, and Javascript).
So, if it that simple, can you do this?
i have no idea how to intergrate such thing.
Well, I think it should require Javascript to hide the categories so that people without Javascript can still see the forum. Or, use the onclick handler on the tabs, and use the href to go to a different category view.
Bump
Well, if you don't have JavaScript on, that looks very... interesting...