I moved your post into the Debian area since Ubuntu is not mandrake but a Debian child. I actually have a laptop with that exact chipset (well each of my kids have a laptop with that chipset). I had to use ndiswrapper + XP driver + wpa_supplicant and they work just fine on FC5 and FC6 (see
this thread) and KNOPPIX so I am sure you should have no trouble getting this working on Ubuntu. I don't have Ubuntu installed on either of those laptops so I can't give you exact instructions but if nobody here has an answer soon I will install Ubuntu on my son's laptop and get a step by step for you. Come to think of it I have a Netgear USB wireless card that I think has the same chipset that I could use in a VMware session. I'll still have to install Ubuntu in VMware though.
EDIT: This looks like it should work for getting your broadcom driver installed and working:
http://ubuntuforums.org/showthread.php?t=25683
Then you have to get your wpa_supplicant configured. Are you using WPA? That's what I use. Here is a wpa_supplicant document for Ubunut:
https://help.ubuntu.com/community/WifiDocs/WPAHowTo
You may have already been through those.
EDIT2: Well I installed Ubuntu 6.06 in a VMware session but I realized my Netgear USB wireless card was actually Atheros based so it uses a different XP driver in ndis. I figured that would be no problem, everything still should have been about the same but I don't think this card is going to work at all in a VMware session. I got it to show up with an "lsusb" and I got the driver installed but the wlan0 device doesn't show up in iwconfig so the driver obviously isn't talking to the card. So, I'll have to actually make room on my son's laptop and install it there. I might not have time to do that for a couple of days though and if I can't get to it before Thursday it will be next week before I can get to it (the son and I are going riding/camping in KY). Hopefully you can get it going.