My netbook is connected to an external display at times. I run lucid Ubuntu netbook edition as my window manager. I believe this bug shares some similarities and may even be a dupe of bug 713922, but the effects are fairly different and thus for now I stick to "one issue, one report".
When running dual-head and the internal, smaller display is positioned below the bigger, external display maxizing a window will not take into account the task bar on the smaller display (the GNOME task bar hides the Firefox menu on the maximized window in the lower screen of the attached screenshot). Maximizing a window when the internal display is to the left of the external one works fine.