Bethesda is not known for adding invisible barriers inside the world perimeter, to me they have been more creative than this.
Actually, downtown DC in Fallout 3 has quite a few them, forcing you to use the metro tunnels to get anywhere. I just hope all the mountains in Skyrim don't lead to a similar situation.
The potential problem with caves and mountain passes is that when you funnel the player down a narrow pathway to get from point A to point B, it gets old real fast. How many of you continued to use the metros to reach downtown DC locations after you discovered them? I'll bet very few.
No matter how pretty a mountain pass is, if we're forced into being funneled through the landscape in that way, people are just going to fast travel.
The "fun" in "funneled" is a lie.Note: this is part of why we need a
climbing skill.