If you have upgraded to Mac OS X 10.5.2 and use a MacBook Pro with an external display in closed-lid mode, do not install Leopard Graphics Update 1.0. Once Leopard Graphics Update 1.0 is applied, your MacBook Pro will mistake the external display for a second internal display and it will not function in closed-lid mode.
This appears to be a widespread issue. An official bug report has been filed and Apple may be addressing it soon. For now, you can force your MacBook Pro to use the external display by closing the lid immediately after pressing the power button, before the chime.