C:\Documents and Settings\Administrator\Desktop>java -version
java version "1.6.0_10-beta"
Java(TM) SE Runtime Environment (build 1.6.0_10-beta-b25)
Java HotSpot(TM) Client VM (build 11.0-b12, mixed mode, sharing)


 The exception happens at start-up time when creating the Swing objects in different threads.
 As it's thread related it is not always reproducable but I have it at more than 9/10. Start the application several times then to see it.

 I use Win XP/sp2 with build 1.6.0_10-beta-b25

Start test case over and over again, it may take 10+ times before you see the exception, sooner on a multi-cpu system.

---------------------test case--------------------------------------------
import java.awt.*;
import javax.swing.*;

// javac TestNimbusThreading.java
// java -Dswing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel TestNimbusThreading
public class TestNimbusThreading {

    public TestNimbusThreading() {
        try {
            final JFrame mainFrame = new JFrame();
            final JPanel jpGrid = new JPanel(new GridLayout(5,5));
            final SwingObjectCreation[] tableCreation = new SwingObjectCreation[25];
            for (int i=0; i<tableCreation.length; i++) {
                tableCreation[i] = new SwingObjectCreation();
                tableCreation[i].start();
            }
            for (int i=0; i<tableCreation.length; i++) {
                tableCreation[i].join();
            }
            SwingUtilities.invokeLater(new Runnable() {
                    public void run() {
                        for (int i=0; i<tableCreation.length; i++) {
                            jpGrid.add(new JScrollPane(tableCreation[i].getTable()));
                        }
                        mainFrame.getContentPane().add(jpGrid);
                        mainFrame.pack();
                        mainFrame.setVisible(true);
                        mainFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
                    }
            });
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    
    public static void main(String[] args) {
        new TestNimbusThreading();
    }
    
    public static class SwingObjectCreation extends Thread {

        private JTable table;
        
        public void run() {
            table = new JTable(3,2);
        }
        
        public JTable getTable() {
            return table;
        }
    }
}

---------------------------end test case -------------------------------