Not always, it is possible and when done right it looks incredible.
Arx Fatalis did it well IMO, and had a great (believable) jump animation.
I think that its possible to mod FO3/NV to show the legs, but you would have to edit every outfit & armor in the game.
(Also... It may affect dismemberment if you do (I don't really know). The game makes everything but the arms invisible when in FPP)
This one outfit that I made does show more of the body than just the arms when in FPP.
http://www.youtube.com/watch?v=tTw7M7gAUKc
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Technically you can see your feet, but its not what you think.