WEP
/etc/network/interfaces
auto wlan0 inet dhcp wireless-essid ESSID wireless-key1 s:key wireless-keymode restricted
WPA
wpa_passphrase ESSID >> /etc/wpa_supplicant.conf
/etc/network/interfaces
auto wlan0 iface wlan0 inet dhcp wpa-driver wext wpa-conf /etc/wpa_supplicant.conf