FULL PRODUCT VERSION :
java version "1.6.0_10-rc"
Java(TM) SE Runtime Environment (build 1.6.0_10-rc-b28)
Java HotSpot(TM) Client VM (build 11.0-b15, mixed mode, sharing)

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

A DESCRIPTION OF THE PROBLEM :
After changing the Look & Feel from Metal to Nimbus and back to Metal, the Nimbus font continues to be used by a JFileChooser.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run code.
Close file chooser dialog.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Expect to see the same font both times the Metal L&F is used:

font javax.swing.plaf.FontUIResource[family=Dialog,name=Dialog,style=plain,size=12]
font javax.swing.plaf.FontUIResource[family=SansSerif,name=sansserif,style=plain,size=12]
ffont javax.swing.plaf.FontUIResource[family=Dialog,name=Dialog,style=plain,size=12]

ACTUAL -
font javax.swing.plaf.FontUIResource[family=Dialog,name=Dialog,style=plain,size=12]
font javax.swing.plaf.FontUIResource[family=SansSerif,name=sansserif,style=plain,size=12]
font javax.swing.plaf.FontUIResource[family=SansSerif,name=sansserif,style=plain,size=12]


REPRODUCIBILITY :
This bug can be reproduced always.

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

public class JFileChooserBug10 {

   public static void main(String args[]) {
      try {
         UIManager.setLookAndFeel(
               "javax.swing.plaf.metal.MetalLookAndFeel");
      }
      catch (Exception e) {
      }
      JFileChooser fc = new JFileChooser();
      fc.showOpenDialog(null);
      System.out.println("font " + fc.getFont());
      try {
         UIManager.setLookAndFeel(
               "com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
      }
      catch (Exception e) {
      }
      SwingUtilities.updateComponentTreeUI(fc);
      System.out.println("font " + fc.getFont());
      try {
         UIManager.setLookAndFeel(
               "javax.swing.plaf.metal.MetalLookAndFeel");
      }
      catch (Exception e) {
      }
      SwingUtilities.updateComponentTreeUI(fc);
      System.out.println("font " + fc.getFont());
   }

}


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

CUSTOMER SUBMITTED WORKAROUND :
 Subclass JFileChooser and override updateUI() so that it nulls the font
before calling super.updateUI():

  public void updateUI() {
         setFont(null);
         super.updateUI();
  }