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Debian sarge (amd64) does not come with the software needed to enable WPA authtentication on wireless lans.

before you are tempted to give up and go with WEP security, read about the security flaws here.

Security of the WEP algorithm:
http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html

you will need to download wpa_supplicant. Get the latest release. (Please check for card compatibility)
http://hostap.epitest.fi/wpa_supplicant/

the Broadcom drivers needs to be installed via ndiswrapper. It does not have native drivers.

Now follow this to make wpa talk to ndiswrapper.
How to use WPA with ndiswrapper
http://ndiswrapper.sourceforge.net/mediawiki/index.php/WPA

as stated in the article, you might need to configure pairwise / groupwise negotiation.
I used the config. below

file: "/etc/wpa_supplicant.conf"
network={
ssid="*******"
scan_ssid=1
psk="********"
key_mgmt=WPA-PSK
pairwise=TKIP
group=TKIP
proto=WPA
}

then just two more commands:
(make sure your driver is loaded and configurable via ifconfig
for me ifconfig wlan0 shows up so...)

1. wpa_supplicant -Dndiswrapper -iwlan0 -c/etc/wpa_supplicant.conf

2. usually you might be using DHCP :
dhclient wlan0

Happy Browsing !