Disable auto screen lock after suspend and hibernate
Even if during installation you chose "Login in automatically" Ubuntu will prompt you for a password after starting from suspend (sleep) and hibernate.
To change this behaviour try this:
System > Preferences > Screensaver
Disable "Lock screen when screensaver is active"
If this doesn't work try:
Open terminal (Applications > Accessories > Terminal) and type gconf-editor
navigate to apps > gnome-power-manager > lock and disable all items
If this still doesn't work repeat this but launch gconf-editor with sudo gconf-editor
Even after this Ubuntu may still prompt for a password if you suspend or hibernate using Ubuntu soft power button (top right on the desktop). If you close laptop lid or use CTRL+ALT+DELETE dialog everything works as expected. This is a bug and is discussed in more detail here.
Ubuntu Desktop 10.10
Ubuntu Desktop 11.04