WpRecipes.com - Daily recipes to cook with WordPress

What you have to do is to paste this code into your functions.php file and let WordPress filters do the work for you.

function my_excerpts($content = false) {
        // If is the home page, an archive, or search results
	if(is_front_page() || is_archive() || is_search()) :
		global $post;
		$content = $post->post_excerpt;

	// If an excerpt is set in the Optional Excerpt box
		if($content) :
			$content = apply_filters('the_excerpt', $content);

	// If no excerpt is set
		else :
			$content = $post->post_content;
			$excerpt_length = 55;
			$words = explode(' ', $content, $excerpt_length + 1);
			if(count($words) > $excerpt_length) :
				array_pop($words);
				array_push($words, '...');
				$content = implode(' ', $words);
			endif;
			$content = '<p>' . $content . '</p>';

		endif;
	endif;

// Make sure to return the content
	return $content;

}

add_filter('the_content', 'my_excerpts');

Thanks to Justin Tadlock for this cool snippet!