Dúvida: abrir janela em JPanel

Olá, estou tentando abrir uma janela dentro de um JPanel.

Principal

package View;

public class Principal extends javax.swing.JFrame {

    /** Creates new form Principal */
    public Principal() {
        initComponents();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        painel = new javax.swing.JPanel();
        jMenuBar1 = new javax.swing.JMenuBar();
        jmenu_arquivo = new javax.swing.JMenu();
        jmenuitem_novo = new javax.swing.JMenuItem();
        jmenuitem_sair = new javax.swing.JMenuItem();
        jMenu2 = new javax.swing.JMenu();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        javax.swing.GroupLayout painelLayout = new javax.swing.GroupLayout(painel);
        painel.setLayout(painelLayout);
        painelLayout.setHorizontalGroup(
            painelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 626, Short.MAX_VALUE)
        );
        painelLayout.setVerticalGroup(
            painelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 498, Short.MAX_VALUE)
        );

        jmenu_arquivo.setText("Arquivo");

        jmenuitem_novo.setText("Novo");
        jmenuitem_novo.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jmenuitem_novoActionPerformed(evt);
            }
        });
        jmenu_arquivo.add(jmenuitem_novo);

        jmenuitem_sair.setText("Sair");
        jmenu_arquivo.add(jmenuitem_sair);

        jMenuBar1.add(jmenu_arquivo);

        jMenu2.setText("Edit");
        jMenuBar1.add(jMenu2);

        setJMenuBar(jMenuBar1);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(painel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(painel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );

        pack();
    }// </editor-fold>

    private void jmenuitem_novoActionPerformed(java.awt.event.ActionEvent evt) {
        Cadastro obj_form = new Cadastro();
        painel.add(obj_form);
        obj_form.setVisible(true);
    }

    /**
    * @param args the command line arguments
    */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Principal().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify
    private javax.swing.JMenu jMenu2;
    private javax.swing.JMenuBar jMenuBar1;
    private javax.swing.JMenu jmenu_arquivo;
    private javax.swing.JMenuItem jmenuitem_novo;
    private javax.swing.JMenuItem jmenuitem_sair;
    private javax.swing.JPanel painel;
    // End of variables declaration

}

Cadastro.java


/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/*
 * Cadastro.java
 *
 * Created on 19/10/2010, 19:43:33
 */

package View;

/**
 *
 * @author Mauro
 */
public class Cadastro extends javax.swing.JPanel {

    /** Creates new form Cadastro */
    public Cadastro() {
        initComponents();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        jInternalFrame1 = new javax.swing.JInternalFrame();

        jInternalFrame1.setVisible(true);

        javax.swing.GroupLayout jInternalFrame1Layout = new javax.swing.GroupLayout(jInternalFrame1.getContentPane());
        jInternalFrame1.getContentPane().setLayout(jInternalFrame1Layout);
        jInternalFrame1Layout.setHorizontalGroup(
            jInternalFrame1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 394, Short.MAX_VALUE)
        );
        jInternalFrame1Layout.setVerticalGroup(
            jInternalFrame1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 274, Short.MAX_VALUE)
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jInternalFrame1)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jInternalFrame1)
        );
    }// </editor-fold>


    // Variables declaration - do not modify
    private javax.swing.JInternalFrame jInternalFrame1;
    // End of variables declaration

}

Mauroemau, postei um projeto aí que acredito atender o que você precisa.Abra-o no seu NetBeans e analise-o.
[]´s

Valeu muito obrigado