Hoi there.
I found a little, the user_block is used to log directly from the portail (I.E.) but a problem remains : it usesThe problem is that U_PORTAL have SID in it. So, you'll be logged ... But your sid won't change.
To correct that, open templates/default/blocks/user_block_block.tpl
FindReplace with
Open user_block.php
Find Before, add
Have a nice day playing around with Icy Phoenix =).