Not being silly is something that lead designer, Emil Pagliarulo apparently feels very passionately about. This is what he told PC Zone:
There are some people who say that making an 18-rated game, and
especially making a Fallout one, should be about sleeping with the
hooker, then waking up with the venereal disease. A lot of that stuff
personally, to me, veers into being a 12-year old dungeon master.
Some of it feels right and some of it feels wrong. We're appealing
to an adult audience and it's a tough call. I mean, once you have groin
shots you're approaching a level of silliness that, if you're not
careful, can pervade the whole game.
So, there we got it on black and white: there will not be any groin shots in Fallout 3... there will, however, be weapon erosion but that is serious.
Update: It seems that a small article on PC Zone was the source for the quote. Thanks to Killzig and Briosafreak.