The way I do it is to close my MBP, then click my wireless mouse to wake it up while its closed. And Viola, I have an external display running w/o my MBP displays on. Once in this mode, opening your MB shouldn't turn on the display either (as long as its still plugged in to your monitor). If you want both screens on again, press detect displays again.
EDIT: This is all under Leopard.