Before Lion it was possible to run an external display off a
laptop and have the internal display disabled, even if you opened
the lid. This can be useful for a myriad of reasons including
energy saving and better Wi-Fi reception. With Lion the internal
display will always turn on when the lid is opened, even if there
is already an external display connected.
I just ran into this after upgrading my main machine to Lion. A little
nvram command-line jiggery-pokery does the trick.