Not that I know of, no. Because as the first poster wrote, HTTP is stateless, meaning that your server program has no way of knowing what's going on with the client (the browser).

The only way around this is to implement your chat program in Java--the java client can keep an open socket to the server, so when the socket closes, the server knows that the client has "logged off."

Of course, this probably also wouldn't work w/ your text-only browsers. :)