Information from: http://superuser.com/questions/71465/how-to-disable-screen-locking-after-sleep-on-ubuntu-9-10

Step 1

Open the terminal, and type gconf-editor In the window that comes up go to apps > gnome-power-manager > lock On the right, UNCHECK suspend and hibernate and make sure that the use_screensaver_settings is CHECKED

Step 2

Go System > Preferences > Screensaver and uncheck the box that says lock when screensaver is active.