I have a entity with an optional unidirectional one-to-one relationship.

@Entity
@IdClass( value = MbiMsgpa.PK.class )
@Table( name = "mbi_msgpa" )
public class MbiMsgpa implements Serializable, Comparable<MbiMsgpa>
...
/** Relation with MbiMsgpt. */
@OneToOne( cascade = { CascadeType.PERSIST, CascadeType.REMOVE }, fetch = FetchType.LAZY )
@Cascade( value = org.hibernate.annotations.CascadeType.DELETE_ORPHAN )
@NotFound( action = NotFoundAction.IGNORE )
@PrimaryKeyJoinColumns( {
@PrimaryKeyJoinColumn( name = "msghd_serial", referencedColumnName = "msghd_serial" ),
@PrimaryKeyJoinColumn( name = "part_no", referencedColumnName = "part_no" )
} )
private MbiMsgpt mbiMsgpt;
...
}

This worked using JBoss-4.2.4 using these Hibernate components
hibernate-annotations: version="3.2.1.GA"
hibernate-entitymanager: version="3.2.1.GA"
hibernate: version="3.2.4.SP1_CP01-brew"
but migrating to JBoss-6.0.0-SNAPSHOT with
Hibernate Annotations 3.5.5-Final
Hibernate 3.5.5-Final
Hibernate Commons Annotations 3.2.0.Final
Hibernate EntityManager 3.5.5-Final
Hibernate Validator 3.1.0.GA
Hibernate Validator 4.0.2.GA
it fails.

I get this exception if there is no entry in MbiMsgpt:
javax.persistence.EntityNotFoundException : Unable to find biz.mbisoftware.fn.ejb.entity.MbiMsgpt with id [MsghdSerial=7741;PartNo=1;]
java.lang.reflect.InvocationTargetException : null
biz.mbisoftware.common.exceptions.MbiException : java.lang.reflect.InvocationTargetException
biz.mbisoftware.fn.ejb.entity.MbiMsgpt_$$_javassist_155(-1).getAdditionalData
biz.mbisoftware.common.XmlHelper(291).methods2Xml
biz.mbisoftware.common.XmlHelper(370).bean2Xml
biz.mbisoftware.common.XmlHelper(301).methods2Xml
biz.mbisoftware.common.XmlHelper(370).bean2Xml
biz.mbisoftware.common.XmlHelper(123).collectionMethod2Xml
biz.mbisoftware.common.XmlHelper(286).methods2Xml
biz.mbisoftware.common.XmlHelper(368).bean2Xml
biz.mbisoftware.fn.ejb.session.MessageDistributorType(73).buildTheXmlDocument
biz.mbisoftware.fn.ejb.message.core.MessageDistributor(123).onMessage
biz.mbisoftware.common.ParameterInterceptor(108).parameters: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [:1.6.0_21]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) [:1.6.0_21]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) [:1.6.0_21]
at java.lang.reflect.Method.invoke(Method.java:597) [:1.6.0_21]
at biz.mbisoftware.common.XmlHelper.methods2Xml(XmlHelper.java:291) [:]
at biz.mbisoftware.common.XmlHelper.bean2Xml(XmlHelper.java:370) [:]
at biz.mbisoftware.common.XmlHelper.methods2Xml(XmlHelper.java:301) [:]
at biz.mbisoftware.common.XmlHelper.bean2Xml(XmlHelper.java:370) [:]
at biz.mbisoftware.common.XmlHelper.collectionMethod2Xml(XmlHelper.java:123) [:]
at biz.mbisoftware.common.XmlHelper.methods2Xml(XmlHelper.java:286) [:]
at biz.mbisoftware.common.XmlHelper.bean2Xml(XmlHelper.java:368) [:]
at biz.mbisoftware.fn.ejb.session.MessageDistributorType.buildTheXmlDocument(MessageDistributorType.java:73) [:]
at biz.mbisoftware.fn.ejb.message.core.MessageDistributor.onMessage(MessageDistributor.java:123) [:]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [:1.6.0_21]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) [:1.6.0_21]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) [:1.6.0_21]
at java.lang.reflect.Method.invoke(Method.java:597) [:1.6.0_21]
at org.jboss.aop.joinpoint.MethodInvocation.invokeTarget(MethodInvocation.java:122) [jboss-aop.jar:2.2.1.Alpha3]
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:111) [jboss-aop.jar:2.2.1.Alpha3]
at org.jboss.ejb3.interceptors.container.ContainerMethodInvocationWrapper.invokeNext(ContainerMethodInvocationWrapper.java:72) [:1.0.8]
at org.jboss.ejb3.interceptors.aop.InvocationContextInterceptor$InvocationContext.proceed(InvocationContextInterceptor.java:138) [:1.0.8]
at biz.mbisoftware.common.ParameterInterceptor.parameters(ParameterInterceptor.java:108) [:]
at sun.reflect.GeneratedMethodAccessor459.invoke(Unknown Source) [:1.6.0_21]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) [:1.6.0_21]
at java.lang.reflect.Method.invoke(Method.java:597) [:1.6.0_21]
at org.jboss.ejb3.interceptors.aop.EJB3InterceptorInterceptor.invoke(EJB3InterceptorInterceptor.java:83) [:1.0.8]