In this post at the Bethsoft forums, Emil Pagliarulo has cleared up the viewpoint issue for those of you who still weren't aware: an isometric viewpoint is basically impossible in Fallout 3 if you actually want to play the game:
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.