I doubt this is the case. It was stated immediately after that there would be multiple endings. You see, the problem isn't that the definitive ending of fallout wasn't that the game ended, but that it was an unsatisfying ending. You died, your character would never be able to see the results of his actions, and to make it worse, this was solved by applying a DLC which forced the player into siding with the BOS.
In New Vegas, multiple definitive endings could be done very well if they were based off the endings in the first two fallouts, where a slideshow would play and show the results of the choices you made in the game, and the effects they had on other people.
I believe, the right way to go about this "definitive ending" is to keep your character alive (unless there is a specific ending where they die, IE Mass Effect 2) and play various cut scenes showing the results of your actions throughout the mojave wasteland. Like, showing you as a high General Slaver in Ceasar's Legion, or a war hero of the NCR, as well as several other cutscenes detailing the aftermath of other smaller factions in relation to your actions.
THAT would be a satisfying way to do it, and would do WONDERS to replay value. So yeah, I believe that Obsidian will model the multiple definitive endings off of the "results of your actions" endings in the original fallout 1/2.
Who else thinks this would be the good way of doing it?