It's been discussed before already, and it just doesn't seem to serve an actual useful purpose (at least not one that is worth the resource overhead of displaying it).
I am assuming that an AJAX heartbeat isn't being sent out which is understandable. I can agree with the fact that they do take a lot of resources, how about coding a system that checks the last time a user viewed a page (this already exists), if the page was loaded more than X minutes ago then the system counts that as an offline user. This is an important feature and a lot of forums are using it and it would help improve the way users communicate through private messages and also lead to increased sales as consumers are more likely to purchase from users that are online in hope of faster delivery.
If someone wants to know if they are online, they can just click on their username/avatar and the membercard popup shows their last activity.
That too, but I am completely sure that it would improve the forum a whole lot and make it better than it already is. Not only would it improve sales as I earlier mentioned but it would also give the feeling that the forum is alive. Just like the "Members Online Now" function it would help the user realize that other people are online.