I created a password-less account for my Mom, that is, in the Ubuntu Installer (or maybe later, using users-admin, I don't think it really matters) I created an account, inserted no password and checked the "don't ask for a password on login" checkbox. After tweaking the permissions a bit, it works fine for her. The only (quite big) problem is that when the screen is locked, either by locking it manually or after a user-switch done by the fast-user-switch applet, you simply can't unlock it, since it asks for a password that I never set.

 I think that gnome-screensaver simply shouldn't ask for a password for this type of accounts.

After unchecking the "lock screen when screensaver is active" box everything works fine when switching user from the GDM dialog, but it does not when using Ubuntu's fast-user-switch applet, nor when clicking "Lock Screen" in the same applet.

original bug report here:
https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/589299
https://answers.launchpad.net/ubuntu/+source/gnome-screensaver/+question/113394