/**
 * get a short/tiny url
 * @author: René Ade
 * @link: http://www.rene-ade.de/inhalte/php-code-zum-erstellen-einer-tinyurl-ueber-tinyurl-com-api.html
 */
if ( !function_exists('fb_gettinyurl') ) {
    function fb_gettinyurl( $url ) {
        $fp = fopen( 'http://tinyurl.com/api-create.php?url=' . $url, 'r' );
        if ( $fp ) {
            $tinyurl = fgets( $fp );
            if( $tinyurl && !empty($tinyurl) )
                $url = $tinyurl;
            fclose( $fp );
        }
        return $url;
    }
}

<a class="tweetthis" href="http://twitter.com/home?status=<?php echo urlencode( 'Lohnenswert: ' . get_the_title() .' ') . fb_gettinyurl( get_permalink() ); ?>" title="Tweet this">
	<img src="<?php bloginfo('stylesheet_directory'); ?>/images/layout/tweetthis.jpg" border="0" />
</a>