Tested with jdk1.3.0_01-beta_refresh-2 on RedHatLinux6.2J/RedHatLinux6.1Jupdate
The latest bundles are located at
/usr/local/java/jdk1.3.0_01-beta_refresh-2/bundles/linux

java version "1.3.0_01-beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0_01-beta)
Java HotSpot(TM) Client VM (build 1.3.0_01beta, mixed mode)


In Launching ControlPanel FileNotFound Exception occurs as below;

java.lang.ClassNotFoundException: sun.plugin.panel.LinuxPlatform
        at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:297)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:286)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:253)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:313)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:120)
        at sun.plugin.panel.ActivatorPanel.getPlatformObject(ActivatorPanel.java:117)
        at sun.plugin.panel.ActivatorPanel.<init>(ActivatorPanel.java:52)
        at sun.plugin.panel.ControlPanel.init(ControlPanel.java:51)
        at sun.plugin.panel.ControlPanel.main(ControlPanel.java:125)


To reproduce,
1. Install jdk1.3.0_01-beta_refresh-2 on RedHatLinux6.2J
2. Launch ControlPanel ($JAVAHOME/jre/bin/ControlPanel)
     ./ControlPanel

Then, you'll see the exception on both of terminal and Java Plug-in Exception Dialog.
ContolPanel can start inspite of the exception.