As discussed in this thread:
  http://forums.java.net/jive/thread.jspa?threadID=43881&tstart=0

The following test case shows rendering artifacts in non-opaque toplevels
when JInternalFrames are used:
public class NonOpaqueWindowTest2 extends JFrame
{
  public NonOpaqueWindowTest2()
  {
    super();    
    setWindowNonOpaque(this);
    
    JDesktopPane desktop = new JDesktopPane();
    JInternalFrame iFrame = new JInternalFrame("Test", true, true, true, true);
    iFrame.add(new JLabel("internal Frame"));
    iFrame.setBounds(10, 10, 300, 200);
    iFrame.setVisible(true);
    desktop.add(iFrame);    
    getContentPane().add(desktop);
    
    setTitle(getClass().getSimpleName());
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(400, 400);
    setLocationRelativeTo(null);
    setVisible(true);
  }
  
  private void setWindowNonOpaque(Window w)
  {
    try 
    {
      Class<?> c = Class.forName("com.sun.awt.AWTUtilities");
      Method m = c.getMethod("setWindowOpaque", Window.class, boolean.class);
      m.invoke(null, w, false);
    } 
    catch (Exception e) 
    {
      e.printStackTrace();
    }    
  }
 
  public static void main(String[] args) throws Exception
  {
    EventQueue.invokeLater(new Runnable(){
      public void run()
      {
        JFrame.setDefaultLookAndFeelDecorated(true);
        new NonOpaqueWindowTest2();
      }      
    });
  }  
}