It would be a nice feature if you could set your sametime status to invisible. Sometime I want to be logged on and chat to someone but don't want to be hassled by everyone else. I know I can change status to Do Not Disturb but I would rather just be invisible. This would also be useful when doing a little work at night time. I could be logged into sametime and still access sametime bots etc but not be visible to other users.
Skype has this feature and I find I often use it so I can contact some people but not others.
An extension to this idea would be to allow yourself to be visible to only a small select group of people.