CS needs zones and to cross from one zone to another there are portals (portal is a flat plane) for. But it is not the main problem with big forests. It's the graphical lag the main enemy. If you see too many faces at once you are going to experience very low FPS, unless you or on new comp and like 2048MB of memory... unfortunately not everyone are so lucky.
In theory we could have portals of any size (in BD there is a huge portal as the great cave opening starts, i think), but then you would instantly see what is on the other side... tis woud cause graphical lag as there is reasons why you load one and not all maps at once. Thats how i see it at least.
But at the same time i believe it is possible to use tricks (including fog, min/max render dist. smaller zones, but with larger portals, so that you can see ony through first portal and what is after another won't render, -tricks) to experience similiar behaviour, but keep the FPS ar reasonable level. In the end there is noone in PS team who would do it. There is not enough time. It is already hard to make completly new zone and everytime you do so, it is better than another. And in the end the first made zone looks like crap comparing to the new.
Currently someone would have to look at the whole thing and work on it hard to make each consistant with another.
And it may be done with these "tunels" too, so well you won't even realise you are crossing one.
