I don't know if anybody has been following the CMS progress
over at aD very closely, but if you haven't yet read their latest UI
specification, it is well worth the read:
http://developer.arsdigita.com/acs-java/cms/panels/
Sadly, the link to the wireframes doesn't appear to be working at
the moment. At any rate, these specifications look outstand and
provide a good model not only for the CMS UI but also for
thinking about UI in general.