I've just gone to my preferences page at MediaWiki.org (
http://www.mediawiki.org/wiki/Special:Preferences, if you're logged in), and the heading for the 'search in these namespaces by default' box contains the following text:
<br />Search in these namespaces by default:<br /><span style="font-size: 90%; font-weight: normal;"><i>(Note: To be most useful it is advised that you include 'Main', 'Help' and 'Manual' as a minimum.)</i></span>
This used to work fine, but it appears that this setting no longer accepts HTML markup.
This should probably be reverted.