With System 10.1, you could hit the power button then close the lid of your PowerBook and have it use only an external monitor - it would ignore the screen of the notebook. This was nice if, like me, the external monitor and the notebook are not sitting side by side.
When 10.1.5 came out this ability was lost. If you turned on the notebook with an external monitor attached, you got an extended desktop if you wanted it or not. All you could do was turn the brightness down on the PowerBook screen. However, this made it easy to loose your pointer on the blank screen.
With 10.2, things are back to the way they were in the "old" days. Now you can save that flat panel for when you don't have a tube monitor handy, extending the useful life of your PowerBook screen.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020906061310575