Emil Pagliarulo clarifies what you can do in the several viewpoints used in Fallout 3:
Hey all!
Sorry for the confusion. Let me clarify that a bit.
I meant, in general, the game is fully playable in third person. You can run around, adventure, get into combat (“run and gun”) etc. all in third person, and the camera was designed to accommodate that kind of gameplay, unlike the third-person camera in Oblivion, which was more of a “vanity mode.”
You can zoom the camera back pretty far in third-person, but there comes a point where it becomes less and less effective the farther back you pull the camera, just because you’re so damn small and it’s hard to gauge where the crosshair is at that point. So yeah, you can use third-person for combat, but the game wasn’t really designed to be played with the camera pulled ALL the way back, isometric style. That’s more for fun, and to survey the scene. It’s really no different than any game that lets you zoom a third-person camera back.
A couple other points of clarification:
— When you enter dialogue, the camera zooms into first-person.
— When you enter V.A.T.S. the camera zooms into first-person.If you were in third-person when you went into one of those mode, you’re back in third-person when you come out.
Fallout 3: A Post Nuclear Blog 
Black! White! Black! White! No, no, black!
Like I said… I’m just going to have to sit tight and see what pops out after all is said and done.
I’d also like to see what the modding community is going to do with it. I’m honestly more excited to see them get their hands on whatever Bethesda gives us than for the game itself.
that’s kind of disappointing. Is there any reason for this?
Well on the dialog side it’s like having talking heads everywhere, not a bad idea, and since everything will use voice overs it was to be expected.
In VATS it’s the only way to be able to use the system really, without things getting too small to understand the percentages and the position of the body parts.
This is one of the reasons not all of us are designers killzig 🙂 If you think hard enough, you will notice why they took that approach.
just seems like expediency to me.