Check your gconf keys (run gconf-editor tool), especially these:
/apps/panel/global/disable_lock_screen
/apps/panel/global/disable_log_out
/apps/panel/global/locked_down
http://gnomesupport.org/forums/viewtopic.php?t=12469
-------------------------------------
|^^^^^^^^^^^^^^^| ||
|...BEER TRUCK..........| ||'|";, ___.
|_..._..._______===|=||_|__|......, ] -
"(@)'(@)"""**|(@)(@)*** **''(@)