Anonymous board use is now available.
If for some reason you would like your online status to be hidden from other users, simply go to your profile page, go to "account related settings", then uncheck the box next to "Show others your online status?"
If you do that, your name will no longer appear in the "Who is online" section at the bottom of the main page while you are browsing the board. You will be listed as one of the "hidden" users.