how to limit search to post titles?

Here’s a filter that’ll do the trick. Drop it into your theme’s functions.php or a plugin.

/**
 * Search SQL filter for matching against post title only.
 *
 * @link    http://wordpress.stackexchange.com/a/11826/1685
 *
 * @param   string      $search
 * @param   WP_Query    $wp_query
 */
function wpse_11826_search_by_title( $search, $wp_query ) {
    if ( ! empty( $search ) && ! empty( $wp_query->query_vars['search_terms'] ) ) {
        global $wpdb;

        $q = $wp_query->query_vars;
        $n = ! empty( $q['exact'] ) ? '' : '%';

        $search = array();

        foreach ( ( array ) $q['search_terms'] as $term )
            $search[] = $wpdb->prepare( "$wpdb->posts.post_title LIKE %s", $n . $wpdb->esc_like( $term ) . $n );

        if ( ! is_user_logged_in() )
            $search[] = "$wpdb->posts.post_password = ''";

        $search=" AND " . implode( ' AND ', $search );
    }

    return $search;
}

add_filter( 'posts_search', 'wpse_11826_search_by_title', 10, 2 );

Most of the code is just copied from the WP_Query class, except with the post_content LIKE‘s removed.

UPDATE: Removed deprecated like_escape() since 4.0