I usually use my 770 with the cover on. I did notice that it sees lots of weak signals that I can't connect to. I thought this was because the AP was less sensitive than my 770. It may be that the cover was compromising my transmit power.

I just did an informal test with my 770 and found that removing the cover caused many signals with one bar to increase to two bars. They went back to one bar when I put the cover back on.

I think you're absolutely correct that the cover acts as a faraday cage. At least the remedy for this problem is simple.