The client side pack 200 support introduced a "jnlp.packEnabled" property. In java plugin, sun.plugin.PluginURLJarFileCallBack does not have the privelege to read this property. We need either read permission for those properties or use AccessController.doPrivileged() to read them.
One testcase is the SwingSet2 applet demo. http://java.sun.com/products/plugin/1.5.0/demos/jfc/SwingSet2/SwingSet2.html
Click the JEditorPane HTML demo, click the "Title Page" link, you will see the exception, and the page is not loaded.