NOTE: See CSS revision 1.2 for completely updated CSS and admin.php
I'm sorry about the poor CSS execution on this first version. I copied it from another site I do (iaslash), but it's not quite flexible enough for our Drupal Admin needs. I worked on a modification quickly a few days ago and am still looking at it. It would look something like this:
I think a 2 column layout would be simpler for handling the
breadcrumbs, should the height of the top box get too tall. The font sizes are bumped down a bit in the navigation and the ul bullets in the left hand nav are removed to conserve space. We might want to look at how those lists are being constructed, because there are a lot of extraneous ul tags embedded that will make it hard for us to controll with CSS.
There's still a lot to do regarding the structure and labeling of components in the admin pages. I would suggest that we not forget the usability recommendations/observations made in the following posts, as some finer points have been raised there and still need to be addressed. I still plan to do a heuristic evaluation in January as time permits.