I still have seen nothing to support the existence of render occlusion culling in Skyrim, which is a shame because the outdated "fog draw distance" method hasn't been used by most developers since like 2005. Fallout 3 had portals, yes, but they had to be manually placed. By contrast, our dear beloved http://en.wikipedia.org/wiki/Quake_engine has been automatically calculating visibility areas since 1996. Okay, so Quake uses BSP map architecture and not meshes.
BSP trees doesn't really work with the way the game world is built. If you build a game map in Quake or most level editors, the editor then have to "render" the level, save the BSP info, save the pre-calculated lightning and so on. That's not the case with MW or OB, nothing is pre-calculated at all, the whole game world is dynamic in that sense.
I think it was a perfectly valid trade-off, going with better and easier to develop content over render speed, when dealing with such large game worlds. Which also proven to be a very good thing for the modding community.
But we have middleware like Umbra now which is wholly capable of serving the same purpose.
It would be nice if Skyrim used Umbra, yeah. Only question is if the whole plug-in infrastructure of the engine allow something like it, or if they had to rebuild it from scratch (which I'm not sure they would bother to do).