A certain Google Maps API requires that an additional xmlns attributed be added to the HTML header tag so that Internet Explorer will render it correctly (sigh). While the Theme has control over HTML headers for external viewing, wp-admin/admin-header.php has control over HTML headers for the admin screens, and there is no namespace hook. (The API is being used in both admin and external screens.)
So, I would like to request a hook be added. Patch coming shortly.
I have tested that no behavior changes in 2.1.2 and [5176] when my hook is added, and that my plugin can use the hook to add a namespace. I also verified it was a new hook name.