Thematic Theme override example for custom 404 pages.

gistfile1.php

// reference http://thematictheme.com/forums/topic/custom-404/

// override 404 content
function childtheme_override_404_content() {
    thematic_postheader(); ?>
    <div class="entry-content">
        <p>Super Awesome Customizations</p>
        <p><?php _e( 'Apologies, but we were unable to find what you were looking for. Perhaps searching will help.', 'thematic' ) ?></p>
    </div><!-- .entry-content -->

    <form id="error404-searchform" method="get" action="<?php echo home_url(); ?>/">
        <div>
            <input id="error404-s" name="s" type="text" value="<?php the_search_query(); ?>" size="40" />
            <input id="error404-searchsubmit" name="searchsubmit" type="submit" value="<?php esc_attr_e( 'Find', 'thematic' ); ?>" />
        </div>
    </form>
<?php } // end 404_content

Related: Changing the title that appears on 404 pages https://gist.github.com/scottnix/8051116