Well, after some more google I found the following solutions:
1. gdmflexiserver: When run from a logged in user it spawns a new login screen.
Good points: A new login gdm console are opened only when requested by the user
Bad points: you must use gdm, you must have a logged in user to run the command, 3D acceleration is only available for the first user
2. editing /etc/X11/gdm/gdm.conf and under the section [servers] Write another two ones:
Good points: doesn't need a logged in user to open a new gnome-session
Bad points: the login gdm consoles will always be opened waiting for user imput
I'm still not satisfied by these options. I really liked the M$ XP implementation of the fast user switching, and I'm looking for something similar.