Are they going to scale to you, a la Oblivion/Fallout 3, or be determined by region, a la FNV? Does anyone know this? If not, what did people prefer?

I hated the level scaling system in Oblivion, as it didn't also apply to town guards and main characters. Martin kept getting smushed by dadrea at the battle of the Great Gate. Maybe if it applied to everyone, not just enemies, it might be better.
At the same time, get munched on by Deathclaws was not too fun, because I didn't know which areas had them or not, or what weapons worked best. Still, it felt good to blow the heads off them after I eventually could.

The previews and interviews claim that some of the enemies will scale to the player, but some certain areas will have preset mobs to make it a bad idea to go there at lower levels.

Each type of enemy (bandit, animal, daedra etc.) is assigned a level range. While you are below that range, that type of enemies is always stronger than you. While you're within that rage, they scale with you. Once you've surpassed the range, they stop scaling and you get more and more powerful compared to them. The ranges vary by minimum level and breadth, so some opponents will scale for a lot longer than others.

Hearing things from hands-on reports I think it's a mix between leveling with you and having a set level.

I've heard about "easily taking on 3 bandits at once" and people getting their asses kicked by necromancers. The latter I've seen in some off-screen footage and looked like a real challenge. I think most of us will be satisfied!