I have just done a site where the client didn't want people to see the session id.....so I created a framed page (I only needed 1) where one frame was hidden. From then on every new page loaded in the frameset that was viewable.
Because it used frames.....the URL never changed.
<frameset rows="100%,*">
<frame src="main_page.php3">
<frame src="blank.html">
</frameset>
You'd never ever know the difference..it worked great.....just don't specify any target in your urls.
rgds,
scott d~