I have a Dell 1548 with the same wi-fi card.
I don't know if you have tried it, go to System<Administration<Hardware Drivers and activate the driver for wi-fi. I know, it's not a open source driver and that.
If you do not have Gnome 2.X, just put this in terminal
sudo aptitude install bcmwl-kernel-source
Hope it works! (it worked for mine)