When I was upgrading some laptops to Ubuntu 11.04 Natty there were no wifi drivers for broadcom 43xx installed afterwards. I had to use a cable to get into the internet.
To solve this problem I installed three packages and got the wireless cards back to work.
apt-get install b43-fwcutter bcmwl-kernel-source firmware-b43-installer
During the installation the required modules are built. Test the wireless connection after
If your wifi works now you can make the module be loaded every startup by appending
echo b43 >> /etc/modules