I just updated my Ubuntu 12.10 today and it went fine and I was told to restart system and after I restart my WiFi stopped working. Mine is Broadcom BCM4312 card . It was working fine just before the update, I tried reinstalling driver from software sources and it didn’t work, it shows the proprietary driver in use but network manager doesn’t detect my WiFi card.
I just fixed the issue, I found the kernel sources has not been installed which prevented from compiling the driver properly. So I installed kernel source for the current kernel from Synaptic then issued the following commands and my WiFi was instantly restored (like 3 or 5 seconds).
To install kernel headers and source:
sudo apt-get install linux linux-headers-generic kernel-package sudo apt-get purge b43-fwcutter firmware-b43-installer firmware-b43-lpphy-installer firmware-b43legacy-installer bcmwl* sudo apt-get install b43-fwcutter firmware-b43-lpphy-installer bcmwl*
This applies to BCM4312 only (My laptop is Lenovo g450)
For others can see this blog post, this might be useful for you.