At least with Java 6u13 (and apparently regardless of platform), executing
  public final class EditorTest {
    public static void main(String[] arguments) {
      Thread.currentThread().setContextClassLoader(null);
      new javax.swing.JEditorPane("text/plain", "");
    }
  }
fails with
  Exception in thread "main" java.lang.NullPointerException
  at java.util.Hashtable.put(Hashtable.java:394)
  at javax.swing.JEditorPane.registerEditorKitForContentType(JEditorPane.java:1329)
  at javax.swing.JEditorPane.registerEditorKitForContentType(JEditorPane.java:1311)
  at javax.swing.JEditorPane.loadDefaultKitsIfNecessary(JEditorPane.java:1389)
  at javax.swing.JEditorPane.getKitTypeRegistry(JEditorPane.java:1346)
  at javax.swing.JEditorPane.createEditorKitForContentType(JEditorPane.java:1271)
  at javax.swing.JEditorPane.getEditorKitForContentType(JEditorPane.java:1128)
  at javax.swing.JEditorPane.setContentType(JEditorPane.java:1019)
  at javax.swing.JEditorPane.<init>(JEditorPane.java:274)
  at EditorTest.main(EditorTest.java:4)

The problem appears to be that the null context class loader shall be put as a value into a Hashtable, which fails with a NullPointerException. The documentation of JEditorPane does not directly state any requirements on the context class loader, and esp. for using the default "text/plain" it is not obvious why there should be any.

That a thread's context class loader is null does indeed happen "in real life," namely when a thread that was spawned outside the JVM is attached to the JVM via JNI; according to <http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/invocation.html#attach_current_thread> "[w]hen a thread is attached to the VM, the context class loader is the bootstrap loader," and in turn "[s]ome implementations may use null to represent the bootstrap class loader," according to <http://java.sun.com/javase/6/docs/api/java/lang/ClassLoader.html#getParent()&gt;).

OpenOffice.org is having a problem with this, see <http://qa.openoffice.org/issues/show_bug.cgi?id=102164#desc8>.