Auto-login could be considered a severe security issue, despite this, sometimes, it could be useful for specific tasks.
To do this job you have only to change a file.
- Gain root privilegies.
- open /etc/gdm/custom.conf with your favourite editor
- after [daemon] line insert the following two lines:
AutomaticLoginEnable=true
AutomaticLogin=username
where username is the user you want to auto-login
Gg1