Adding an external link icon to entry titles

external-icon.css

/* show a little arrow image */
.icon-external { 
    display: inline!important; 
    float: none!important; margin: 0 0 0 4px!important; 
    vertical-align: baseline!important; 
}

wp-external-link-titles.php

<?php

// THIS IS ASSUMING YOU HAVE AN ICON IMAGE IN YOUR THEME!

// any post titles that don't link internally get an icon
add_filter( 'the_title', 'the_title_add_external_icon' );
function the_title_add_external_icon( $title ) {
	if ( get_post_type() != 'link' && ! strstr( get_permalink(), get_home_url() ) )
		$title .= '<img class="icon-external" src="' . get_stylesheet_directory_uri() . '/gfx/icon-external.png" title="External link" alt="(External link)" />';
	return $title;
}

// adds strip tags to esc_attr - get_the_title() is used in an attribute 
// it MUST be wrapped in esc_attr() or esc_attr_e() etc...
add_filter( 'attribute_escape', 'attribute_escape_tags', 9, 2 );
function attribute_escape_tags( $safe_text, $text ) {
	$safe_text = strip_tags( $text );
	$safe_text = wp_check_invalid_utf8( $safe_text );
	$safe_text = _wp_specialchars( $safe_text, ENT_QUOTES );
	return $safe_text;
}

?>