Looks like there is a bug with BigInteger deserialize. Though this has been fixed for Merlin, it still exists in JDK 1.3 and is a regression.
This only occurs when you subclass BigInteger to another class, say UnsignedInt64. When you try to serialize and then deserialize, there is a NullPointerException. If, however, it is merely embedded in a class (and not subclassed), there is no problem. This works fine on 1.2.2_05
I'm attaching two example files. Just run java TestClient to see the result. Our RMI server fails because of this.
Here is a stack trace:
java.lang.NullPointerException
        at java.math.BigInteger.readObject(BigInteger.java:2638)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.io.ObjectInputStream.invokeObjectReader(ObjectInputStream.java:2213)
        at java.io.ObjectInputStream.inputObject(ObjectInputStream.java:1410)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:386)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:236)
        at TestClient.deserialize(TestClient.java:20)
        at TestClient.main(TestClient.java:8)