Many laptops with Broadcom Corporation BCM4312 802.11b/g hardware shows no response to wireless network with Ubuntu 9.10 Casper.
I could solve this problem with the following method:
a) Go to System-> Administration->Hardware Drivers
Keep your system on Internet(wired ethernet) while doing this.
Install Broadcom 43 and Broadcom STA driver, activate them then.
b) Reboot and see if it work for you.
c) If not, then do the following:
sudo apt-get --purge remove bcmwl-kernel-source
sudo apt-get install patch
sudo apt-get install bcmwl-kernel-source
This should solve the matter. At least it worked for me :)
Ref: Ubuntu Forum