FULL PRODUCT VERSION :
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Linux olgas 2.6.18-3-686 #1 SMP Mon Dec 4 16:41:14 UTC 2006 i686 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
JSeparator always draws horizontal separators under the GTK look and feel, even when setting a vertical separator. This affects the separator generated by JToolbar.addSeparator() as well. Please compile and run the enclosed code to reproduce the problem.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Vertical separator.
ACTUAL -
Horizontal separator.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package bugs;

import java.awt.Dimension;

import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSeparator;
import javax.swing.SwingConstants;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;

/**
 * Demonstrate that JSeparator can't display vertical separators under the GTK
 * look and feel in Java 6.
 * <p>
 * The first frame shows a nice vertical separator between the words "foo" and
 * "bar". The second frame shows a horizontal separator at the top of the frame
 * stretching between the words "foo" and "bar."
 *
 * @author Bill Wohler <###@###.###>
 */
public class GtkVerticalSeparatorBug extends JFrame {
    public GtkVerticalSeparatorBug() {
        JPanel panel = new JPanel();
        BoxLayout layout = new BoxLayout(panel, SwingConstants.HORIZONTAL);
        panel.setLayout(layout);

        JLabel foo = new JLabel("foo");
        panel.add(foo);

        JSeparator separator = new JSeparator(SwingConstants.VERTICAL);
        panel.add(separator);

        JLabel bar = new JLabel("bar");
        panel.add(bar);

        add(panel);
        panel.setPreferredSize(new Dimension(300, 20));
        pack();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new GtkVerticalSeparatorBug().setVisible(true);
                try {
                    UIManager
                        .setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel");
                } catch (Exception e) {
                    e.printStackTrace();
                }
                new GtkVerticalSeparatorBug().setVisible(true);
            }
        });
    }
}

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

CUSTOMER SUBMITTED WORKAROUND :
Here's a quick and ugly hack: Use a panel with an etched border. If you can suggest a better workaround (like subclassing JSeparator() and providing a better paint method), I would appreciate hearing about it.

        JPanel separator = new JPanel();
        separator.setBorder(BorderFactory.createEtchedBorder());
        Dimension d = separator.getPreferredSize();
        separator.setMinimumSize(new Dimension(2, d.height));
        separator.setPreferredSize(new Dimension(2, d.height));
        separator.setMaximumSize(new Dimension(2, d.height));