There's a detailed answer to the question at Login without a password. Here's a summary:

First, do you have cookies enabled in your browser?

Another possibility is that you logged on, selected a perm cookie, and then later logged out. Logging out does not remove your name from the list of users. Rather, it removes the cookie (temp or perm). Your name will appear in the list of other users for a few minutes after your last action. Thus you might not be listed as logged in when you are in fact just making a lengthy post.

I think these are the most common reasons--the term 'log out' doesn't completly suggest the consequences. If neither of these is the answer, try seeing Corion's reply to the aformentioned node.