Tested on: WordPress 3.0.1

function content_only4logged_in($content) {

    // ALL LOGGED IN USERS
    if ( is_user_logged_in() &&
            !is_null($content) &&
            !is_feed()
         ) {
        return $content;
    } else {
        $content  = wp_html_excerpt( $content, 80 );
        $content .= ' …';
        $content .= __( 'Sorry, more of this content is only available for logged users.', FB_TEXTDOMAIN );
        return $content;
    }
}
add_action( 'the_content', 'content_only4logged_in' );

more possibilities and informations on the post http://wpengineer.com/2046/control-the-wordpress-content-via-userrights-or-time/