Hi,
I don't know why I wasn't able to do it, but adding:
if [[ $(xrandr|grep '^VGA.*disconnected.*') == '' ]]; then
exit 0
fi
in front of /etc/acpi/actions/lid.sh does the trick (of not going suspended,
when there's an external monitor dangling on your VGA).
Have fun with it (and of course I would like to see it in main ;)),
Malte