I just picked up a mini-DVI to VGA adapter so I can use my 17" desktop LCD display with my MacBook. The idea was to set my laptop in front of my monitor where a keyboard would normally be, so I can use both the keyboard and the screen at a comfortable height. My desktop screen is high enough that it clears the laptop screen.
What I expected to be able to do was direct the laptop screen output to the desktop monitor, complete with menu bar and desktop icons and all my open windows, and turn off the laptop screen so it's just black. In reality, there appear to be two choices: mirrored, or additional desktop. The mirrored mode just sends the same 1280x800 output to the desktop monitor, using only 78% of its 1280x1024 available area. Or, I can use the desktop monitor as additional desktop area at its full 1280x1024, but then I don't get the menu bar or my desktop icons or the ⌘-Tab icon display, or any other stuff that I don't specifically drag up to the desktop monitor.
Is there a way to ... never mind, I just found it. If I drag the menu bar in the display arrangement preferences panel to the top display, then all the desktop icons and dashboard stuff and whatnot goes with it (except the dock, apparently). The desktop display becomes the primary display. If I go mobile and unplug the desktop monitor, everything migrates back to the laptop display. This will work.
Clearly, I made the rational choice.