Adding this PHP code to the functions.php of your wordpress theme will allow you to replace the default wordpress login logo.

replace_login_logo.php

<?php

function loginLogo() {
    echo '<style type="text/css">
        h1 a { background-image:url('.get_bloginfo('template_directory').'path/to/logo.gif) !important; }
    </style>';
}
add_action('login_head', 'loginLogo');

?>