Sure, there are applets and other GUI tools to do this, but nothing beats the purity of going commando, er, commandline. :) Thanks to this article for the inspiration.
# reset
xrandr --auto
# turn off laptop screen
xrandr --output LVDS --off
# 24" 16x12 on
xrandr --output VGA --mode 1600x1200
# reset
xrandr --auto
# turn off external screen
xrandr --output VGA --off
# laptop screen on
xrandr --output LVDS --mode 1280x800
UPDATE, 2010/01/05:
To enable both screens and force the larger one to include the taskbar:
# reset
xrandr --auto
# laptop screen on
xrandr --output LVDS1 --mode 1280x800 --noprimary --pos 1600x0
# 24" 16x12 on
xrandr --output VGA1 --mode 1600x1200 --primary --pos 0x0
# echo current status
xrandr --current