Hello,
then you also need to tell the system to connect to the network:
wpa_supplicant -Dwext -iwlan0 -c /etc/wpa_supplicant/wpa_supplicant.conf
You might also want to automatically reconnect to the network, this can be done using the provided script:
Go to /etc/ifplugd/action.d/ and rename the ifupdown file to ifupdown.original
Then do: cp /etc/wpa_supplicant/ifupdown.sh ./ifupdown