Web accelerators have been doing this for years.
The programs cache all the info you send then threads it in whenever you re-visit or someone else revisits the page you see. But since it uses ALL the info you send it thinks someone else is YOU. Back a few years ago i was in this clan, it was a big clan 50+ members, and one of our admins was at a friends and checked the site under his admin account. His friend had dial up and had d/l'ed an accelerator, he didnt know at the time, so it gave out his account, some punk found it, got in, hit reset on our clan website and PUFF all gone. Why he had access to a site wide reset button i will never know.