I use this configuration every day at work. You can also drag the white line representing the menu bar to the external monitor if you want. This makes more sense if your external display is the top monitor, which is probably the case.
When you plug/unplug the external monitor, the PB will automatically figure out what to do and reposition the menu bar and desktop icons.
The only thing to watch for is if you close an application which has a window open on the external display, then unplug the display, and then restart the application. In some cases the window opens where the screen used to be, so you can't see it on the main PB screen. This is a real nuisance as I haven't found a good way to access the window to drag it onto the main PB screen. You can see it in Expose, but can't grab it, DOH! Some applications seem to figure out that there's no screen attached any more, and reposition the window where you can get to it, but others are oblivious.
