I know that this is possible, I've seen it done, but I've forgotten the command.
Basically, what I want to do (for various reasons) is to spawn a GDM/Gnome/etc session (a new one) from within my current session. I don't want to use another tty. I want it to be windowed, and appear under my current session, so I can easily Alt+Tab, start/stop the session easily.
Ideally I'll be able to do something to the effect of: