The bug reveals the URL of the page a user is viewing to the Web server of the site visited last. This allows a Web server to track where users go after they leave the site, even if the next Web address comes from a bookmark or is manually typed into the browser.
Also present in Netscape 7.0 and Galeon. <sigh>......

http://news.zdnet.co.uk/story/0,,t269-s2122261,00.html

Demonstration of the vulnerability