FULL PRODUCT VERSION :
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-b05)
Java HotSpot(TM) Client VM (build 1.5.0_06-b05, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
JRootPane does not respect the opaque property.
Doing a setOpaque(false) on the JRootPane will result of an opaque component because the content pane is leaved opaque.

The javadoc suggest that the content pane of a JRootPane is expected to be the root component of a swing hierarchy. It is generally the case but it should not be assumed.

One could extends JRootPane to create components using the glass pane functionnality of the JRootPane. In that case, making the JRootPane not opaque should make the content pane automatically not opaque.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a JRootPane, set it opaque to false.
Insert a non opaque JLabel in the JRootPane.
Insert the JRootPane in a Root container witch have a different background color.
See source code...



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
You should not see the JRootPane but see the JLabel with the background color set to the Root container:
ACTUAL -
The JRootPane is shown with a gray background.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.Color;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JRootPane;

public class JRootPaneAlwaysOpaque{

   public static void main(String[] args) {
      JFrame frame = new JFrame();
      frame.getContentPane().setBackground(Color.BLUE);

      JRootPane rootPane = new JRootPane();
      rootPane.setOpaque(false);

      JLabel lbl = new JLabel("The background is supposed to be blue");

      frame.getContentPane().add(rootPane);
      rootPane.getContentPane().add(lbl);

      frame.setSize(500,50);
      frame.setVisible(true);
   }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
You can manually force the content pane to be transparent this way:

((JComponent)rootPane.getContentPane()).setOpaque(false);

The cast is ugly and this is why I think it is a bug in the current implementation of a JRootPane.
Again, I think the JRootPane should hide the implementation of the content pane (exposing only the Container interface as it is now) but should also copy it's opaque attribute ot the content pane.