FULL PRODUCT VERSION :
java version "1.5.0_08"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_08-b03)
Java HotSpot(TM) Client VM (build 1.5.0_08-b03, mixed mode)

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

A DESCRIPTION OF THE PROBLEM :
On jdk1.5_08, when using WindowXP look and feel calling JToolBar.setOpaque(false) has no affect. This is a regression as it worked as expected in jdk1.5_07.

This works as expected for all other look and feels I tested: Metal, Motif, and windows classic.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a JPanel and set it's background color so Color.Red
2. Set the toolbar's opaque property to false - JToolBar.setOpaque(false)
3. Place the JToolBar on JPanel


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Background of JPanel should show through the JToolBar background.
ACTUAL -
Background of JPanel does not show through the JToolBar background.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.*;
import javax.swing.*;

public class ToolBarBugDemo
{
	public static void main(String[] args) throws Exception
	{
	    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

		JFrame frame = new JFrame();
	    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	    frame.setSize(new Dimension(400, 300));
	    frame.setLocationRelativeTo(null);

		JToolBar toolBar = new JToolBar();
		toolBar.setFloatable(false);
		toolBar.setOpaque(false);
		
		toolBar.add(new JButton("New"));
		toolBar.add(new JButton("Open"));
		toolBar.add(new JButton("Close"));
		
		frame.getContentPane().setBackground(Color.RED);
	    frame.getContentPane().add(toolBar, BorderLayout.NORTH);

	    frame.setVisible(true);
	}
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None that I know of. In my case my panels background image is a gradient that needs to show through the toolbar background.

Release Regression From : 5.0u7
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.