Firstly, thanks Richard for the register_globals contribution :) You're great!
Anyway, I also had the same logout session_unregister problem for the catalog.. Not sure if I'm doing the right thing, but I followed your instructions and suggestions and got mine working finally.
In the logout.php file, I added this:
tep_session_destroy();
on the line just above:
require(DIR_WS_INCLUDES . 'application_bottom.php');
It worked but as I'm a php newbie, so please correct me if I'm wrong.
Thanks again :)