It looks like it was added at 2005-03-02 and thus you might have to use the cvs version of hibernate3.
{{{
im@linTs1:~/projects/Hibernate3> grep -ir SerializableClob *
Übereinstimmungen in Binärdatei build/classes/org/hibernate/lob/SerializableClob.class.
Übereinstimmungen in Binärdatei build/classes/org/hibernate/type/ClobType.class.
Übereinstimmungen in Binärdatei classes/org/hibernate/type/ClobType.class.
Übereinstimmungen in Binärdatei classes/org/hibernate/lob/SerializableClob.class.
src/org/hibernate/lob/CVS/Entries:/SerializableClob.java/1.1/Wed Mar 02 11:43:35 2005//
src/org/hibernate/lob/SerializableClob.
java://$Id: SerializableClob.java,v 1.1 2005/03/02 11:43:35 oneovthafew Exp $
src/org/hibernate/lob/SerializableClob.java:public class SerializableClob implements Serializable, Clob {
src/org/hibernate/lob/SerializableClob.java: public SerializableClob(Clob blob) {
src/org/hibernate/type/ClobType.java:import org.hibernate.lob.SerializableClob;
src/org/hibernate/type/ClobType.java: return rs.wasNull() ? null : new SerializableClob(value);
im@linTs1:~/projects/Hibernate3>
}}}