On serveral different WiFi networks I have experienced "Unable to join the netork" errors. It seems to be one of two problems. The first, is that I am getting terrible signal strength. I am less than 20 ft from my router, my laptop shows full reception, my iphone shows 1 bar. I try to connect repeatedly and it gives me the unable to connect error. Have others experienced this bad of reception or is my iPhone defective? Secondly, even if I get really close to the router, and can get 2-3 bars for WiFi, I still frequently get the unable to connect error. Anyone experiencing this? Any idea how to fix it?

Sounds like a hardware problem to me. There is a small flex cable inside the iPhone which connects the main board to the bezel (which is used an an antenna).
If the screw is loose or the flex has somehow become torn or if the connector has detached itself from the main board, this will result in low signal. It should be a cheap repair considering the flex costs less than a fiver. My bet is that connector (3) has become loose after a sharp knock to the iPhone.