Ubuntu unveils its next-generation shell and display server

I don't. At least not without some kind of overlay (*). The minute a normal application can go fullscreen it can spoof security measures (for instance, ask the user for his password and store/send it somewhere).

(*) One possible solution here is an OSD overlay that can be shown for a limited time, like two to five seconds, after any input -- keystroke, mouse move &c and, after that, vanish (like youtube's and xbmc's fullscreen mode video controls). This way the user can watch a video or even read a book or a webpage using the whole screen estate but can't be deceived in thinking its login page &c is being displayed.