This error has been recorded many times before as a Google search will show, but all those reports were for earlier versions of Ubuntu whereas I use 9.10.
There were a lot of updates for indicator applets yesterday, and this error showed up after the first reboot after these updates were installed. (Please go through the attached file from dpkg.log for the files that were installed.)The panel encountered a problem while loading "OAFIID:_FastUserSwitchApplet.
Delete Don't Delete.
Oh, and the packages gnome-applets and gdm are "already installed and the newest versions."