You got a typo on one of the settings in the Edit Features and Options section - "Show online/fffline in posts and PMs"
fffline? :P
http://www.simplemachines.org/community/index.php?topic=10228.0
http://www.simplemachines.org/community/index.php?topic=11482.0
http://www.simplemachines.org/community/index.php?topic=10184.0
i know, i thought i discovered it too :P
hrm... I guess I didn't look as hard as I thought :P
at least you werent the only one who did that ;)