crwdns2893792:0crwdne2893792:0 Dan ,


The back cover itself does have an impact on reception, as it acts as a ground plane for the motherboard.  I have seen iPhone 4's and 4S' alike with serious reception issues simply from not having a good back cover properly installed.  It is not common, most of the time the phone should work normally with no back cover on it whatsoever, but I just wanted to mention that it is a possibility.

Also if you have access to a microscope and multimeter, check continuity of the inductors (small rectangular 2-pin component that is essential a piece of plastic wrapped in wire) near the antenna connector.  If one were knocked loose due to impact, it will also cause this issue.