I hate to bump an old topic, but I gotta tell you guys, you can go to Morrowind in Oblivion. It's nothing at all like the actual Morrowind from the previous Elder Scrolls, but this Youtube video shows a guy who actually got past the "You can't go any farther" barriers. http://www.youtube.com/watch?v=Dd5KcFi5hnw

True, Morrowind on the map is just put there to show relation to the other Elderscroll games, and so it's just an area with lots of trees to keep the world from looking like it really ends (although if you go off the map it kinda does :P). But still, if a mod were to be released anytime soon (seeing as how it's a few years after Oblivion's release date I'm not sure if that will ever happen) I think it would be great to tear down some of the barrier and trees to civilize the land.