Fortunately, Fallout 3 will not use Oblivion’s level-scaling, but contrary to what you may have read elsewhere, scaling isn’t completely discarded.
The first important change is that creatures never scale up in abilities to match your level, so each Deathclaw in F3 will always have the same attributes, regardless of your character’s level when you have the misfortune of encountering it.
Second, each territory in the game is now assigned an encounter level that determines the level and equipment of critters when you discover that area, so a first-level character that wanders into an area designated as “encounter level 5” will be badly outmatched by the inhabitants. Loot is also generally scaled to the area’s encounter level, but some item items will be hand-placed, which is similar to how Morrowind handled loot.
An area’s level doesn’t remain static, but it gets locked as soon as you enter it. If you enter a city block designated as a level 5 area, it will remain a level 5 area and never scale up in difficulty. Areas you haven’t yet encountered do “tether up” in difficulty level, but the tethering level doesn’t linearly scale with your level, so there’s still an advantage to gaining experience levels.
The city block that’s initially designated as a level 5 area will tether up and be designated as a level8 area if you don’t wander into it until you’re a level 15 character. But since an area’s level is locked once you enter it, you’ll still get the satisfaction of returning to a previously difficult area and annihilating its residents once you have a more powerful character.
Bethesda’s still tweaking these systems, but they should make exploration more interesting and not diminish the regard for advancement by making you feel like you can never really get ahead. I’d still prefer a static world like Gothic’s, where encounters are always consistent regardless of your character level, but this toned-down scaling system sounds like a huge improvement over Oblivions.