The top node will always intersect the frustum as it contains all of your geometry.
I'm a little bored and this is only a minor correction to the otherwise excellent explanation above:
The top node will of course not always intersect the frustum, but it should always be evaluated in order for one to determine whether anything in the tree is visible. Obviously, mhagain suggests that if we need to check if a state is one of the 3 mentioned states, the root node could be set to the intersect state before every tree evaluation, to simplify things.