It's been a while since I played, but the Vault has it pretty spot on:
Sometimes, when you enter an area, the enemies in it will be scaled to your level. This, however, occurs in a wide range, so the enemies could still destroy you or be easy pickings. This also only occurred once, the enemies would not scale to you any further after that. If you come back later in the game, they're still the same level they were as the first time you encountered them.
Certain places are always inhabited by powerful enemies: Vault 87 always had Supermuties, and Old Olney always has deathclaws. As you level up, you are more prone to run into powerful enemies, but only in new areas you haven't visited yet. The enemies in the main quest are usually scaled around your level, to keep the main quest a bit challenging.
Correct me if anything's wrong.