I've got an old Dell Inspiron 1300 that is equipped with a Broadcom wifi that worked fine until I made an upgrade of the kernel recently. I believe it was when I switched to kernel 2.6.28-14 that something was messed up. The card worked alright but I could not authenticate against my wireless router anymore after the kernel upgrade. The thing is that all traffic between the client and the router is encrypted using WPA-PSK (Wi-Fi Protected Access Pre-Shared Key) and I suspect that it was here something failed, but I may be mistaken. Anyway this how I solved the problem. 

First thing, check the hardware. This is what is reported about the wifi card from lspci:

02:03.0 Network controller: Broadcom Corporation BCM4318 [AirForce One 54g] 802.11g Wireless LAN Controller (rev 02)

To get the card going again I installed the Broadcom firmware cutter package:

sudo aptitude install b43-fwcutter 

Now I just had to run a script that is shipped with the package:

sudo /usr/share/b43-fwcutter/install_bcm43xx_firmware.sh 

One can read more about the firmware cutter here: 

http://linuxwireless.org/en/users/Drivers/b43 [1]