After serializing an object on 1.6 with the code (WriteTest.java, attached), deserializing the object with the code (ReadTest.java, also attached) on the prior releases (say 1.4.1) will throw the following exception:

java.io.StreamCorruptedException
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1301)
        at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1845)
        at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1769)
        at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:324)
        at ReadTest.<init>(ReadTest.java:13)
        at ReadTest.main(ReadTest.java:6)

According to the serialization specification, adding a field is a compatible change:
http://java.sun.com/j2se/1.5.0/docs/guide/serialization/spec/version.html#6519

That the new field uses a class that's not known to an old JRE, RoundingMode in this case, shouldn't matter because deserialization should just skip over the data for the unknown field.
###@###.### 2005-03-14 19:14:33 GMT
###@###.### 2005-03-14 19:16:54 GMT