WPA:

apt-get install wpasupplicant

/etc/network/interfaces:

auto wlan0
iface wlan0 inet dhcp
    wpa-ssid YOUR-SSID-HERE
    wpa-psk YOUR-PASSWORD-HERE

wpa_passphrase SSID > wpa.conf

wpa_supplicant -Dwext -ieth1 -c/root/wpa.conf

-B background