I believe this was a security feature that was added in later versions of magento. It appears to have been added in version 1.7, see login.phtml
If you inspect the login form
<form method="post" action="" id="loginForm" autocomplete="off">
This will try to prevent forms from auto-completing fields if they support the functionality. You can read more about the functionality here and here.
If you really wanted to turn it back on it should be possible to update the template in a custom admin theme or update the name of the template file via a custom layout XML file.
This change in the form is considered a Good Thing (TM). Its a security risk to have backend users saving their passwords in their browser for an application such as Magento where sensitive information is stored.