Hook into `the_post` – called when the post is actually _used_ – and fill the title. Be aware the slug has to be changed too. Fixing just the title is rather pointless.  

If you are used not to enter a title, hook into `save_post` too, and let the same code do this for you.

## The code

[Download on GitHub][1]

    <?php
    /**
     * Plugin Name: T5 Lazy Title Updates
     * Description: Fill missing post titles from content when a post is called
     * Plugin URI:
     * Version:     2013.01.08
     * Author:      Thomas Scholz
     * Author URI:  http://toscho.de
     * Licence:     MIT
     * License URI: http://opensource.org/licenses/MIT
     */
    
    add_action( 'the_post', 't5_lazy_title_update' );
    add_action( 'save_post', 't5_lazy_title_update_save', 10, 2 );
    
    /**
     * Add atitle when the post is called in setup_postdata().
     *
     * @wp-hook the_post
     * @param   object $post
     * @return  void
     */
    function t5_lazy_title_update( $post )
    {
    	if ( ! empty ( $post->post_title )
    		or '' === $clean_content = wp_strip_all_tags( $post->post_content, TRUE )
    		)
    		return;
    
    	$words = preg_split( "/\s+/", $clean_content, 40, PREG_SPLIT_NO_EMPTY );
    	$title = implode( ' ', $words );
    	$title = rtrim( $title, '.;,*' );
    	$slug  = wp_unique_post_slug(
    		sanitize_title_with_dashes( $title ),
    		$post->ID,
    		$post->post_status,
    		$post->post_type,
    		$post->post_parent
    	);
    
    	wp_update_post(
    		array (
    			'ID'         => $post->ID,
    			'post_title' => $title,
    			'post_name'  => $slug
    		)
    	);
    
    	// $post is passed by reference, so we update this property in realtime
    	$post->post_title = $title;
    	$post->post_name  = $slug;
    }
    
    /**
     * Fill title from post content on save
     *
     * @wp-hook save_post
     * @param   int $post_ID
     * @param   object $post
     * @return  void;
     */
    function t5_lazy_title_update_save( $post_ID, $post )
    {
    	t5_lazy_title_update( $post );
    }


  [1]: https://gist.github.com/4480763