It is a requirement of any good JavaBean, according to the JavaBean spec, to implement Serializable. Hibernate won't cough up locally if you don't explicitly add the 'implements' but you might run into problems later on when you go into production and start passing around entities between cluster members.
Hey, sticks and stones baby. And maybe a wee mention of my stuff:
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop