I had the same problem after replacing the screen on my iphone 4. I didn't even notice that I'd lost the little metal piece that fits around the long screw on the bottom of the wifi shield. I took the advice from Dave above and put a small square of tin foil on top of the logic board right where that log screw goes in (under the wifi shield). When the wifi shield is put back on, the long screw is driven through the tin foil which acts like the missing metal piece. Worked like a charm! Just make sure the piece of tin foil is just big enough to fit in place of the missing metal part and does not touch the nearby connection points (the little metal dots) on the logic board below it. If you're not sure, check out the picture of the bottom of the shield that a couple people posted above.
