Well, I think it should be like Oblivion's system, but improved with more natural barriers and lore-based barriers that make sense. For example, bridges that have been destroyed over chasms due to the civil war, mountain passes closed off by rubble, rockfalls, or avalanches, and of course some huge mountains with natural steep cliffs that are part of the normal geography.
So basically, it should be broken down like this. First, here's the http://i.imgur.com/nxx36.png. Now, from looking at that, we can determine how the borders should work.
Out of the 100% of border area, we can assume roughly 30% to be the sea to the north. That border could work simply as an endless sea with no profit for swimming further out, or an invisible barrier far out, or sea serpents or something. Next, we can conclude based on the known geography that almost 40% of the remaining land border is a string of naturally mountainous terrain that is incredibly difficult if not impossible to traverse (this is fitting to the lore for Skyrim as well, since most national borders throughout history IRL, and in Tamriel, are determined by natural barriers and terrain/biome fluctuations as borders). Finally, we can estimate that the remaining space made up of mountain passes, lowlands, and generally "normal" terrain between Skyrim and the surrounding provinces can be covered with about 10% of that space being impassable due to lore-related circumstances and the remaining 20% left to being wilderness that has to have a "turn back" invisible barrier.
Granted, those are rough estimates, and since I nor anyone here knows the exact layout of Skyrim's borders, we can only speculate. But based on the evidence we have, I'd say Bethesda has it easier making borders for Skyrim than for Cyrodiil. And even with that 1/5 of the map area with invisible barriers in the wilderness, I really don't think anyone will care. I mean, who really gets upset when they reach the obvious map edge? Why would you care about it when you could just turn around and go play the game?
I think It will be the best map border design yet, given the geography.