There is a new feature called 'Who is On Line?'. If you click on the link you can see a list of who is on line and what topic they are reading, starting or replying to or which forum they are browsing.
Many of the the people browsing or reading are either not registered or registered but not logged in and they will appear as 'Guests'. When you log in you are also given the option to remain 'visible' or 'invisible' in the list of registered users in the 'Who is On Line' page.
Just gives you the option to a bit more privacy if you are that concerned about it.
For those who are not familiar with the Who is On Line feature
Click Here