This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
It simply means you cant serialize a class using (for example) java 1.3.1 and deserialize it in java 1.4.2
You can deserialize it using java 1.3.1 only. This applies for any other build of java 1.3 too. You cant deserialize using java 1.3.2 ( or something like that ) since the serial version UID would be different.