The one thing I've heard (with mixed results) was to reset your network settings. In the Settings app, General, Reset, Reset Network Settings. This will erase any saved networks (you'll need the WEP/WPA keys and/or settings to re-add), but hopefully that's all you'll lose. I think there was another thing to try with an app in Cydia, but hopefully someone more knowledgeable about that will chime in soon.
Not sure about this but it seems that I have heard of people having to do something in terms of a work around to get the wi-fi working with a JB. To the OP check out the JB forums and you may find the answer that you are looking for.
I am JB and I have never had an issue with WiFi. I did have to do a reset like mentioned above to get my 3G Data flowing. I had full 3G signal and my phone wouldnt load anything online. So I reset the network settings and all was well again. Give that a shot and see if it works.
