I recently came across this problem working on a webapp and deploying to JBoss4.0.1/Tomcat5.0 running on Java SE 5.0 – it seems that in 5.0 the JVM strictly enforces a verison value for each serializable class, and if one is not explicitly declared then it is generated, and chances are the value will be different each time a webapp is deployed, even if there are no changes to the Serializable classes.
See my post here in my Java notes section on how to resolve this.