I've got my WPC54GS Linksys wifi card, which uses a closed-source chipset (Broadcom), working with Slackware 10.2 installed on a Dell Inspiron 8500. I've posted the process to get this working before at slackwiki.org.
I've scripted this process. You can grab the script from here. Edit it as you see fit. Many people actually use the tools that come with Slackware (the wireless configs in /etc/rc.d, I believe). I script my own, as I've many different wifi cards that I often swap out for different needs.
Anyways, give it a shot.
My next task is getting WPA working with the card (wpa_supplicant, I believe).