Because projector usually have a different resolution then high resolution laptop LCDs one has to change the resultion of the X Server after connecting the projector. This used to require an X server restart. With RandR 1.2 this is possible without. It does not work with Xgl though. Thus to use compiz with it one has to use Aiglx (which in turn requires to change the video plug-ins). One can set the resolution either with xrandr or more comfortable with krandrtray under KDE.