In the original Fallouts I mostly just aimed for the eyes and groin like I assume most people did after building their skills up, although to be fair crippling actually did something in the first two games and I did find myself aiming for limbs now and then, in fact during the Horrigan fight in Fo2 I found it *absofrickinlutely* necessary to cripple his limbs or I just couldn't win.
Fallout 3 on the other hand makes aiming for limbs practically worthless, yep there's a minor effect but it's so damn miniscule you might as well just always go for the head...which I did, over and over, until I found a mod called Amplified Crippled Effects which made things work more like you'd expect them to.
I've got my fingers crossed that they sort this out in New Vegas