here you go

dbus-send --system --type=method_call --dest=com.nokia.mce /com/nokia/mce/request com.nokia.mce.request.req_tklock_mode_change string:"locked" | echo ""

i got it from http://wiki.maemo.org/Desktop_Comman...creen_and_keys

but make sure you change settings so its like below otherwise it will lock everytime you goto homescreen lol