Our example: we try to set the default value for a filter on a certain week.
I tried to set the default value of your exposed filter in:
<?php
$form['week']['#default_value'];
?>
Didnt work.
In get forms you need to set the $form_state['input']['week']. Also you need to make sure the value isnt submitted yet so check on $_GET['week'] to get the actual value.
