If you stay away from your computer under Ubuntu, you will find when you return that your screen is automatically locked. To unlock it, you have to submit your root password.
To disable screen locking on Ubuntu 11.04 (also applicable on Ubuntu 10.04/10.10), then follow these simple instructions:
1. Press Alt+F2 and search for "gconf-editor".
2. Open this path:
desktop > Gnome > lockdown
3. On the right side of the window, check on "disable_lock_screen", then close the window.
4. From now on, your screen will no longer get locked automatically in case of system inactivity.