Every now and again someone calls me for support regarding their screensaver, usually after playing around with the settings... To save you some time, heres a mini howto:
The settings are all stored in a database, to view the database and descriptions of each key, run the command: gconf-editor
The actual screensaver settings for Ubuntu 10.04.3 LTS:
gconftool --set /apps/gnome-screensaver/lock_enabled --type bool 0
gconftool --set /apps/gnome-screensaver/idle_activation_enabled --type bool 1
gconftool --set /apps/gnome-screensaver/mode --type string random
The config above also helps avoid that auto-lock screen feature after installing Ubuntu and is perfect for those preseed installs too!