Last time I checked, this sets the defaults only for the first time a user logs in. If a user has logged in previously, you need to update the saved preferences via database script as mentioned early in this thread.
Also you should probably be modifying LocalSettings.php rather than includes/DefaultSettings.php....
DanB
-----Original Message----- From: mediawiki-l-bounces@lists.wikimedia.org [mailto:mediawiki-l-bounces@lists.wikimedia.org] On Behalf Of Adam Meyer Sent: Thursday, August 02, 2007 11:47 AM To: MediaWiki announcements and site admin list Subject: Re: [Mediawiki-l] Changing Default Search Parameters
in includes/DefaultSettings.php
search for NS_MAIN => true,
should look like
$wgNamespacesToBeSearchedDefault = array( NS_MAIN => true,
add this right under it.
NS_CATEGORY => true