Dig the shnazzy MDI window manager UI on IntelliJ! It has two great features that would be wonderful for general window managing as well as for MDI.
The first is the ability to keep windows automatically tiled; growing one window shrinks the other, such that their borders stay stuck together. In today's world of big monitors and bigger resolutions, most simultaneous window work is best done with all windows completely visible. (For example, when I edit code in one window, consult a reference in another, and discuss it over IM in a third, it's now sensible to have no overlapping windows.) This style of MDI has none of the disadvantages of the traditional "window within a window" style; it's very clear what's going on, and windows never get "lost". It even handles minimizing one of the windows nicely, causing the other window(s) to grow to fill the newly available space. Maybe "maximize" could be overriden to give a tabbed interface :).
The second great feature is iconifying windows to fixed locations at the borders of the main window. This is wonderful, because unlike the taskbar model, it capitalizes on spatial memory. (e.g. My log viewing application is always the blue on an inch below the upper left corner of the screen.) You don't have to consciously remember where the representation of each task is, because your muscles will develop the habit of finding it. Of course, "corners of the screen" is just as good as "corners of the main window", and you could implement hiding behaviour so that these icons are aren't visible (or as large) until you mouse-over them.
Just like tab support in the window manager, this might be tough to implement. Anybody take that as a challenge?