Hibernate version:
Hibernate 3 Current CVS

Mapping documents:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>

<class
name="wdb.Zeiteinheit"
table="ZEITEINHEIT">

<composite-id
name="key"
class="tools.BigDecimalId">
<key-property
name="oid"
type="big_decimal"
column="zeiteinheitid"/>
<key-property
name="classtype"
type="string"
length="32"
column="class_type"/>
</composite-id>
</class>

</hibernate-mapping>

Code between sessionFactory.openSession() and session.close():
Object temp = session.get(Zeiteinheit.class, new BigDecimal(1));
ze = (Zeiteinheit)temp;

Full stack trace of any exception that occurs:
org.hibernate.PropertyAccessException: exception getting property value with CGLIB (set hibernate.cglib.use_reflection_optimizer=false for more info) getter of tools.BigDecimalId.?
at org.hibernate.tuple.PojoComponentTuplizer.getPropertyValues(PojoComponentTuplizer.java:79)
at org.hibernate.type.ComponentType.getPropertyValues(ComponentType.java:307)
at org.hibernate.type.ComponentType.getHashCode(ComponentType.java:158)
at org.hibernate.engine.EntityKey.getHashCode(EntityKey.java:68)
at org.hibernate.engine.EntityKey.<init>(EntityKey.java:41)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:72)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:634)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:627)
at wdb.Test.testKurs(Test.java:59)
at wdb.Test.main(Test.java:32)
Caused by: java.lang.ClassCastException
at tools.BigDecimalId$$BulkBeanByCGLIB$$a916dd38.getPropertyValues(<generated>)
at net.sf.cglib.beans.BulkBean.getPropertyValues(BulkBean.java:48)
at org.hibernate.tuple.PojoComponentTuplizer.getPropertyValues(PojoComponentTuplizer.java:76)
... 9 more
Exception in thread "main"

and with hibernate.cglib.use_reflection_optimizer false

ERROR - IllegalArgumentException in class: tools.BigDecimalId, getter method of property: oid
org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of tools.BigDecimalId.oid
at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:171)
at org.hibernate.tuple.AbstractComponentTuplizer.getPropertyValue(AbstractComponentTuplizer.java:121)
at org.hibernate.tuple.AbstractComponentTuplizer.getPropertyValues(AbstractComponentTuplizer.java:133)
at org.hibernate.tuple.PojoComponentTuplizer.getPropertyValues(PojoComponentTuplizer.java:88)
at org.hibernate.type.ComponentType.getPropertyValues(ComponentType.java:307)
at org.hibernate.type.ComponentType.getHashCode(ComponentType.java:158)
at org.hibernate.engine.EntityKey.getHashCode(EntityKey.java:68)
at org.hibernate.engine.EntityKey.<init>(EntityKey.java:41)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:72)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:634)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:627)
at wdb.Test.testKurs(Test.java:59)
at wdb.Test.main(Test.java:32)
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:145)
... 12 more
Exception in thread "main"

Name and version of the database you are using:
Oracle 7.3.3.0.0

The generated SQL (show_sql=true):
nothing

tools.BigDecimalId:
package tools;

import java.math.BigDecimal;

public class BigDecimalId implements java.io.Serializable {
protected String classtype;
protected BigDecimal oid;

public BigDecimalId() {
super();
oid = new BigDecimal(0);
classtype = "";
}

public BigDecimalId(BigDecimal id, String classtype) {
super();
setOid(id);
setClasstype(classtype);
}

public BigDecimal getOid() {
return oid;
}

public void setOid(BigDecimal c_id) {
this.oid = c_id;
}

public String getClasstype() {
return classtype;
}

public void setClasstype(String classtype) {
this.classtype = classtype;
}

public int hashCode() {
return getOid().hashCode() + getClasstype().hashCode();
}
public boolean equals(Object other) {
if (other instanceof BigDecimalId) {
BigDecimalId that = (BigDecimalId) other;
return that.getOid().equals(getOid()) &&
that.getClasstype().equals(getClasstype());
}
else {
return false;
}
}
}

Any hints?

Maybe I'm not following what you're trying to do here, but shouldn't your code look more like this: