About once a day, usually in the morning, my F16 desktop becomes unresponsive until I switch to the command line console and do a "killall gnome-shell". I had the same issue with F15. When I move the mouse to the upper left corner of the screen, the corner lights up but then nothing else happens. I can still move the mouse, and running applications continue to generate output, but the system does not respond to any mouse actions until gnome-shell is restarted. There doesn't appear to be anything in the logs other than a long history of identical messages from dbus PackageKit and NetworkManager, which is disabled.
Thanks for any ideas.