Jailbroken iPhone 3G Wi-Fi is greyed out, no 3G, need to open a mobileconfig file or load pan editing
I have heard many times of wifi greyed out, and generally it was right after updating their iOS with a new version. No amount of DFU seemed to fix it, and I think it is because the issue was with the firmware killing the iPhone's wifi.

I am fairly certain Apple repairs these types of problems, you might even call them and see if they will replace it as their update possibly caused it. 

For the second piece, If you are trying to get a network connection without wifi, prepare for some fun. It looks like you will need to run some sort of proxy service on your computer, modify some plists, and then run a few commands.
The first link has some more mac os x specific stuff, but they are both worth a read.

http://alk2blog.blogspot.com/2008/08/ssh-to-get-your-iphone-online-via-usb.html
http://modmyi.com/forums/general-iphone-chat/323281-how-internet-iphone-through-usb-no-wifi-no-edge.html