Reproducible on Solaris & XP. The following code cannot be compiled. Compiler complains NimbusLookAndFeel cannot be found.

import sun.swing.plaf.nimbus.NimbusLookAndFeel;
import com.sun.java.swing.plaf.windows.WindowsLookAndFeel;
import com.sun.java.swing.plaf.motif.MotifLookAndFeel;
import javax.swing.*;

public class NimbusTest extends JFrame {

  public NimbusTest() {
    try {
      UIManager.setLookAndFeel(new NimbusLookAndFeel());
      setDefaultCloseOperation(EXIT_ON_CLOSE);

      JButton b1= new JButton();
      b1.setText("Button 1");
      getContentPane().add(b1, java.awt.BorderLayout.CENTER);

      pack();

    } catch (UnsupportedLookAndFeelException e) {
      e.printStackTrace();
    }
  }

  public static void main(String[] args) {
    java.awt.EventQueue.invokeLater(new Runnable() {
      public void run() {
        new NimbusTest().setVisible(true);
      }
    });
  }
}

Error message:
NimbusTest.java:3: package sun.swing.plaf.nimbus does not exist
import sun.swing.plaf.nimbus.NimbusLookAndFeel;
                            ^
NimbusTest.java:12: cannot find symbol
symbol : class NimbusLookAndFeel
location: class NimbusTest
      UIManager.setLookAndFeel(new NimbusLookAndFeel());

This is reproducible with 6u5b04 & b05.