John will probably have to answer definitively, but I believe that it knows when you last visited by baking a cookie on your machine. So if you clear your cookies, or don't allow the cookie to be baked onto your local machine, the feature won't work.
There must be something stored on the server as I bob on here from both my laptop (usually at work.. ssshhhh!) and main machine at home and the latest posts always keeps in sync.. Could be fluke but I don't think so..
The mySQL/Postrgress connection seems a little glitchy today as well (loads of errors popping up)