Sorry to bring this thread up again but i am having a similar issue under Windows XP (didnt verify Vista)
When maximizing an MDI Child there is a small line apperaing between the application workspace and the ribbon.
It appears to me this is the standard titlebar of the window.
The Problem is on this titlebar there are still buttons for maximize and close etc. Note that even a tooltip is shown for the minimize button
See Picture in attachements.