FULL PRODUCT VERSION :
java version "1.6.0_10-beta"
Java(TM) SE Runtime Environment (build 1.6.0_10-beta-b23)
Java HotSpot(TM) Client VM (build 11.0-b11, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Windows XP

A DESCRIPTION OF THE PROBLEM :
Trying to set the default font size of tab text in a JTabbedPane via the UIDefaults causes text to extend outside the bounds of the tab. Also, tabs erratically resize when adjusting the size of the window and the selected tab has a smaller font than the non-selected tabs.


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.BorderLayout;
import java.awt.Font;
import java.awt.GridBagLayout;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTabbedPane;
import javax.swing.UIDefaults;
import javax.swing.UIManager;
import javax.swing.plaf.FontUIResource;

public class TabsFrame extends JFrame {

    public static void main(String args[]) {
        try {
            UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
            UIDefaults defaults = UIManager.getLookAndFeelDefaults();
            Font font = defaults.getFont("TabbedPane.font");
            font = font.deriveFont(16f);
            defaults.put("TabbedPane.font", new FontUIResource(font));

            TabsFrame frame = new TabsFrame();
            frame.setVisible(true);
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * Create the frame
     */
    public TabsFrame() {
        setBounds(100, 100, 500, 375);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final JTabbedPane tabbedPane = new JTabbedPane();
        getContentPane().add(tabbedPane, BorderLayout.CENTER);

        final JPanel panel = new JPanel();
        panel.setLayout(new BorderLayout());
        tabbedPane.addTab("This is a tab", null, panel, null);

        final JPanel panel_1 = new JPanel();
        panel_1.setLayout(new GridBagLayout());
        tabbedPane.addTab("Tab2", null, panel_1, null);

        final JPanel panel_2 = new JPanel();
        tabbedPane.addTab("This will be tab 3", null, panel_2, null);
    }
}

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

CUSTOMER SUBMITTED WORKAROUND :
Set the font of each individal JTabbedPane instead of using the UIDefaults to set all JTabbedPane instances.