I am experiencing the same issue. I can't help you fix it, but maybe shed some light on why it is happening. Are you you using a DSL router/gateway? I'm using an Actiontec GT724WGR. If I forget the network in WiFI setting, the signal for my network goes to excellent. As soon as I try to connect to the network, the signal drops to poor, therefore not connecting. After some digging, apparently Motorola did not include support for certain routers. My Actiontec being on that list. Would anyone know if the WiFI system files on JB could be replaced with WiFi files from ICS? I assume you would have to have root to do this.
Didn't mean to hijack you thread, just thought I could shed some light on the problem. Here is a list of routers I have found to be incompatible at this time, as per Motorola:
Westell VersaLink 327W
Actiontec Gt724-WG
Netgear DG834GT
Netgear DG934G (Sky Version)
Lynksys WRTP54G
Motorola Netopia 3347-02
Siemens gigaset SE567
Actiontec GT701 -WG
I did try to search for update firmware for my router, with no luck as Actiontec no longer supports the model. I also did a factory reset on the phone, just for giggles. No luck there either.