I just preupgraded another system to F15 and see the very same problem there. Also, I noticed that "log out" shows the same issue, so this is very likely a gnome-shell (or at least desktop session) related problem. Any idea what files I need to clean or something like that?

It looks like tomboy (which has its only-style try icon sitting in the notification bar) is somehow blocking the shutdown process. Is there anything gnome-shell can do to better work around broken apps?

Can confirm this. Give Gnote a try.

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