To get a blog displaying the Search widget to validate with WebAIM's [URL=http://wave.webaim.org/]WAVE validation tool[/URL], the following change needs to be made:

inc/widgets/widgets/_coll_search_form.widget.php
line 89:

-'defaultvalue' => T_('Search'),
+'defaultvalue' => '<label for="search">' . T_('Search') . '</label>',

line 123:

-echo '<input type="text" name="s" size="25" value="'.htmlspecialchars($s).'" class="SearchField" />';
+echo '<input type="text" id="search" name="s" size="25" value="'.htmlspecialchars($s).'" class="SearchField" />';