Here is how I use my PB15 and my external 19" LCD.
1. I have the LCD plugged into the PB, but you can plug it in after OSX is up.
2. With the PB lid open, start OSX (i.e., turn it on 🙂 )
3. Now that OSX is up, and you have a picture on both screens, close the lid to your PB. This will put your PB to sleep. mmmmm, pretty glowing light.
4. Now, if you have bluetooth, wake your system up (with the lid closed) with your bluetooth device. I have a keyboard, and mouse, so I just move the mouse around, or press a button on the keyboard. This will wake up your PB, and display the primary on your external LCD.
6. Now that you are up and running, open the PB lid, and it will stay off. At least id does on mine.
Make sure you go into the bluetooth settings area, and click the button to allow bluetooth devices to wake the computer system.
I have been doing this since March, and have never had a problem.
Kimo