login_headertitle
login_headertitle is a filter that filters the title for the link at the top of the login page. Use it to change the title.
Context:
File: wp-login.php
<div id="login"><h1><a href="<?php echo apply_filters('login_headerurl', 'http://wordpress.org/'); ?>" title="<?php echo apply_filters('login_headertitle', __('Powered by WordPress')); ?>"><span class="hide"><?php bloginfo('name'); ?></span></a></h1>
This hook is a filter which means that information is passed through it, and then used by WordPress. Your function needs to accept that information, and return it. Using add_filter('login_headertitle', 'your_function'); helps you to remember this distinction. When you are passing an ID, it is assumed that you will return the ID as it was given to you. With filters that pass strings or arrays, you may manipulate the information before passing it along.
This hook was introduced in WordPress 2.1, and will not work in earlier versions.