If you're using Karmic Koala (or Lucid Lynx), the boot loader, Grub will seem a little bit different from previous Ubuntu releases. Previously to make changes like the timeout before loading the default OS, you edited /boot/grub/menu.lst

Grub is now configured by editing (as a root user)

1.)  >  sudo nano /etc/default/grub

Make changes to the GRUB_TIMEOUT or whatever else you needed to change. The config now needs to be made live. Do this by calling

2.) >   sudo update-grub

The changes are now written to an auto-generated file /boot/grub/grub.cfg but this file shouldn't be edited by hand.

For more info, see the Ubuntu's official docs on the subject.