• detect the numbers of monitors you have attached
• Activate the second (external) monitor if there are two monitors
• Use the external monitory as the primary display, otherwise use internal display as a primary display
• Move your taskbars to the primary display

It is based on disper tool by Willem van Engen. For now (2009), it’s nVidia only. ATI support could be possible.

#!/bin/sh
#
# Detect displays and move panels to the primary display
#
# Copyright 2009 Twinapex Research
#
# Author <mikko.ohtamaa@twinapex.com>
#

# disper command will detect and configure monitors
disper --displays=auto -e

# parse output from disper tool how many displays we have attached
# disper prints 2 lines per displer
lines=`disper -l|wc -l`

display_count=$((lines / 2))

echo $display_count

echo "Detected display count:" $display_count

# Make sure that we move panels to the correct display based
# on the display count
if [ $display_count = 1 ] ; then
   echo "Moving panels to the internal LCD display"
   gconftool-2 \
        --set "/apps/panel/toplevels/bottom_panel_screen0/monitor" \
        --type integer "0"
   gconftool-2 \
        --set "/apps/panel/toplevels/top_panel_screen0/monitor" \
        --type integer "0"
else
   echo "Moving panels to the external display"
   gconftool-2 \
        --set "/apps/panel/toplevels/bottom_panel_screen0/monitor" \
        --type integer "1"
   gconftool-2 \
        --set "/apps/panel/toplevels/top_panel_screen0/monitor" \
        --type integer "1"
fi