If you have the >= 2.6.33.x kernel series, that's what you'll do:
1) Download the hybrid-portsrc driver at http://www.broadcom.com/support/802.11/linux_sta.php
(choose either 32-bit or 64-bit, depends on your linux install)
2) Extract the files to a folder called hybrid_wl, for example. This should do:
tar zxf hybrid-portsrc-x86_XX-v220.127.116.11.tar.gz -C hybrid_wl/
Where XX is either 32 or 64.
3) Issue the fallowing terminal command while at the folder 'hybrid_wl/src/include'
$ sed -i '23s/linux\/autoconf.h/generated\/autoconf.h/' linuxver.h
4) Go to hybrid_wl folder and issue the fallowing commands:
$ sudo make install
$ sudo depmod -a
$ sudo modprobe wl
The led must be turned on, showing that the driver was successfully installed.
On step 3), this command will edit the linuxver.h file in order to make it compatible to the kernel >= 2.6.33.x series so it can be built on those kernels. If your kernel is < 2.6.33.x series, just skip step 3) and everything should work fine.
This hybrid driver is not opensource, so it'll turn the tainted kernel flag on, meaning that any debug output for the wireless card will be disabled. If you kernel is >=2.6.29.x series, you should give the b43 driver a try - it's fully supported by the community and is a GPLed driver. For more information, go to: http://wireless.kernel.org/en/users/Drivers/b43