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.