The page you refer mentions JDK 1.3. Are you using that? jBPM won't compile against it.
No, I'm using 1.4.2_05
I've checked the source code of ObjectInputStream for both 1.4.2 & 1.5 and they both make a magic call to a native lastUserDefinedClassloader() call to resolve classes.
I've changed SerializableToByteArrayConverter to use a class like JBoss'
instead of ObjectInputStream and the ClassNotFoundException was gone