Ozh’ Admin Drop Down Menu plugin for wordpress is a great plugin which give you a top menu version for admin navigation.

See http://planetozh.com/blog/my-projects/wordpress-admin-menu-drop-down-css/

However whilst implementing in WordPress 3.2.2  I found an issue with the ability to hide the original wordpress top menu. A fetaure that was previously working.

I worked out these CSS changes to sort that out.  the file to modify is wp-content/plugins/ozh-admin-drop-down-menu/inc/adminmenu.css.php

Around line 308 you will find

<!--?php if ($minimode) { ?-->
/* Hide all header */
#wphead {display:none;}
<!--?php } ?-->

replace that with

<!--?php if ($minimode) { ?-->
/* Hide all header */
#wpadminbar, #wpadminbar * {display:none;}
body.admin-bar #wpcontent {padding-top: 0px;}
#ozhmenu {padding-top: 7px;}
<!--?php } ?-->

That works for me on WordPress 3.2.2 of course future or earlier versions this may not work.
Hope that helps