This guide is half of your solution:
Basically it has you add an option to your GRUB menu that starts the system at a different runlevel. Then you can modify the scripts for that runlevel which will configure the system however you want. However, the second half of the guide only covers modifying xsetup and your network interfaces.
The setting you're looking for is in Power Management. We can use
gconf-editor to find the name of the setting, in this case
/apps/gnome-power-manager/timeout/sleep_display_battery. A setting of 0 will disable the screen shut-off, otherwise it is the number of seconds until shut-off. To change this setting using a runlevel script or the command-line, you can run:
$ gconftool-2 --set "/apps/gnome-power-manager/timeout/sleep_display_battery" --type integer 0
If you're having trouble putting that command and the runlevel guide together, please comment on this and I will try and assist further.