This is an old revision of the document!
/etc/network/interfaces :
auto lo iface lo inet loopback iface eth0 inet dhcp allow-hotplug wlan0 iface wlan0 inet manual wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf iface default inet dhcp
/etc/wpa_supplicant/wpa_supplicant.conf :
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev update_config=1 network={ ssid="OpenWrt" psk="_wallstreet39_" key_mgmt=WPA-PSK }