How can I make the “Preview Post” button save and preview in the same window?

Here’s a way to do it without modifying the core:

add_action('admin_footer','preview_same_window');

function preview_same_window(){ ?>
    <script type="text/javascript">

    jQuery(function($){

     jQuery('.preview.button').unbind().removeAttr('target');

            setTimeout(function(){
                jQuery('.preview.button').unbind().removeAttr('target');
            },250);

    });

    </script>
    <?php
}