Sounds like the airport antenna (wires that run the bottom and side perimeters of the lcd) may have been left disconnected or been cut. My only thoughts are to remove the keyboard (following proper procedure) and confirm that your airport card has its cables connected - usually a white and black clipon pair of wires that lead up to the display. Also there's a possibility that these wires got pinched or severed when replacing the white hinge clutch cover.

Before tearing into your machine like I did, check the brightness control for your screen in system preferences,display, and adjust the brightness slider. This is a common problem and this one minute fix took care of my problem on two computers. Felt stupid, but hey it worked. Sometimes you have to think laterally, instead of linearly.

Chris - I am having the same wifi problems and when I tool my hinge cover back off I noticed that the black wire on the left was severed...Is there a way to get replacement wires for this or do I need to take it in to get it looked at? Thanks!