FULL PRODUCT VERSION :
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_08-b03Java HotSpot(TM) Client VM (build 1.5.0_08-b03, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Windows XP 5.1.2600

EXTRA RELEVANT SYSTEM CONFIGURATION :
Windows XP L&F

A DESCRIPTION OF THE PROBLEM :
The text on JToolBar buttons which are disabled remains black, should go grey. Worked OK in 1.5.0_07 and earlier.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Add a disabled button to a JToolBar under Windows XP using the XP L&F.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Text on disabled buttons should go grey
ACTUAL -
Text on disabled buttons remains black

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
/* ToolBarBugs.java
 * Created on 21 August 2006, 11:27
 */
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.Box;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JToolBar;
import javax.swing.UIManager;
/**
 * @author JohnM
 */
public class ToolBarBugs extends JFrame{
    static final String HTML_TABLE_START = "";     
	static final String HTML_TABLE_END = "";
    
    /** Creates a new instance of ToolBarBugs */
    public ToolBarBugs() {
        super("ToolBar Bugs 1.5.0_08 under XP L&F");
        setLayout(new BorderLayout());
        JPanel p =new JPanel();
        add(new JLabel(HTML_TABLE_START +
                "Illustrates bugs in JToolBar with 1.5.0_08. " +
                "Under the XP Silver L&F the background of the toolbar " +
                "does not match the background of the buttons or the " +
                "background of the frame. \n" +
                "Under any XP std L&F (Blue, silver, Olive) the text of " +
                "disabled buttons does not take on a disabled appearance." +
                "N.B. Must be run on an XP system!" +
                "Check when run under 1.5.0_08 vs 1.5.0_07 or earlier" + HTML_TABLE_END), BorderLayout.CENTER);
        JToolBar t = new JToolBar();
        final JButton b2 = new JButton("I am enabled");
        final JButton b = new JButton("Enabled/Disable other button");
        
		b.addActionListener(new ActionListener(){
			public void actionPerformed(ActionEvent e){
		b2.setEnabled(!b2.isEnabled());
		b2.setText(b2.isEnabled() ? "I am enabled" : "I am disabled");
			}
		});

		t.add(b2);        // make some space between the buttons so easier to see the toolbar background
        t.add(Box.createRigidArea(new Dimension(10,10)));
        t.add(b);
        add(t,BorderLayout.NORTH);
        pack();
	}	    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
		e.printStackTrace();
        }
        final ToolBarBugs bugFrame = new ToolBarBugs();
        bugFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        EventQueue.invokeLater(new Runnable(){
            public void run(){
                bugFrame.setVisible(true);
            }
        });
    }

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Wrap the toolbar button text in HTML tags, subclass JButton and override setEnabled() to force the foreground colour to UIManager.get("Button.disabledForeground")

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.