I liked Oblivion, but I'm wary of a "Oblivion in the Fallout universe" game.
First of all, I was one of those people who liked the turn-based combat. The Oblivion combat was way too easy. So long as you were persistent and had any speed at all, you could take down any opponent with nothing more than a club. The combat only really became fun when I made a stealth character for the sake of bypassing all of the really boring combat (and occasional stealth-kills, those are fun too of course)
In Fallout, every location you could visit had some importance. In Oblivion, there were countless dungeons, caves, landmarks, etc. that were completely meaningless. Some of them were neat the first time you went through, but sadly many of the dungeon designs were repeated. Some of them even had neat backstories and easter eggs. The majority of them made it feel like I had no reason to explore some dimly lit set of caverns, killing all of its inhabitants for phat lewtz.
Fallout 3 has incredible potential, and the graphics already look outstanding. I'm very wary that it won't live up to the first two games, though. Who knows, maybe gameplay of the Oblivion style is just what the franchise needs...