Ok here's what Todd Howard said:
[quote="Todd Howard Executive Producer Fallout 3"]Todd Howard: Being that we are Bethesda…everything gets a bit big. So as of last week we’re over 200 endings. That is not an exaggeration, but it deserves some description. 200 endings…that’s a lot. So originally when we started, we had various iterations of the ending. The ending is kind of cinematic, that’s dynamic based on the things you’ve done.

When we started, it was kind of fuzzy, it was like “well there’s like 9 maybe 12″ and we started adding things to it. So if you had done this or not this, you’d get this other tweak to the ending. And we kept doing that. And you know even just two weeks ago someone had this idea, “Oh we should add this idea to the ending” (sorry I’m not going to spoil what that is). And I said, “oh that’s a genius idea, we have to do that.” But then it became, “oh, but there’s four versions of that.” So i was like, “okay there’s like four different versions of that little parts,” and that multiplies by, at the time we were at about 60 ending…now there’s four versions of that so now theres around 240 versions. So it gets kind of stupid.”

The games are small at first on paper, and then as we go they get bigger…we can’t stop ourselves. We’re have tons of people with good ideas here, and if they’re good and fit the tone, we’re going to try to jam them in there. Fallout is probably twice the size of what we originally had on paper…it’s pretty big, so that’s happened with the endings.

So some of that is the big things of what you do really late in the game, some of its related to karma — how you’ve lived your life from the beginning — you get certain scenes based on your karma. But we kind of like the ending with as much as the game from the beginning — you tailoring your character and then you play throughout this game, and unlike Elder Scrolls where you can keep playing, Fallout 3 has a definite ending. So we wanted to go to efforts to make sure that the actual ending you get when you finish and get the ending, and make that ending reflect and make it individual to the user’s experience.” We’ve got a little overboard.”[/quote]