Hello all,
Starting from the problem discussed in:
http://www.jboss.com/index.html?module=bb&op=viewtopic&t=77402
and
http://www.jboss.com/index.html?module=bb&op=viewtopic&t=77397
about string_max, I included the configuration file in my persistence.xml as shown below:

<entity-manager>
 <persistence-unit name="myUnit">
 <provider>org.hibernate.ejb.HibernatePersistence</provider>
 <jta-data-source>java:myUnitDS</jta-data-source>
 <properties>
 <property name="hibernate.dialect" value="org.hibernate.dialect.PostgreSQLDialect"/>
 <property name="hibernate.hbm2ddl.auto" value="none"/>
 <property name="hibernate.ejb.cfgfile" value="hibernate.queries.hbm.xml"/>
 </properties>
 </persistence-unit>
</entity-manager>

In the begining it did not find the mapping file in the META-INF folder of the .jar, I moved to the root of the jar and it finds the file now but it gives me the following error:

java.lang.reflect.InvocationTargetException
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
 at java.lang.reflect.Method.invoke(Method.java:585)
 at org.jboss.ejb3.ServiceDelegateWrapper.startService(ServiceDelegateWrapper.java:97)
 at org.jboss.system.ServiceMBeanSupport.jbossInternalStart(ServiceMBeanSupport.java:289)
 at org.jboss.system.ServiceMBeanSupport.jbossInternalLifecycle(ServiceMBeanSupport.java:245)
 at sun.reflect.GeneratedMethodAccessor220.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
 at java.lang.reflect.Method.invoke(Method.java:585)
 at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:155)
 at org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
 at org.jboss.mx.server.Invocation.invoke(Invocation.java:86)
 at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:260)
 at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
 at org.jboss.system.ServiceController$ServiceProxy.invoke(ServiceController.java:974)
 at $Proxy0.start(Unknown Source)
 at org.jboss.system.ServiceController.start(ServiceController.java:417)
 at sun.reflect.GeneratedMethodAccessor9.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
 at java.lang.reflect.Method.invoke(Method.java:585)
 at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:155)
 at org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
 at org.jboss.mx.server.Invocation.invoke(Invocation.java:86)
 at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:260)
 at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
 at org.jboss.mx.util.MBeanProxyExt.invoke(MBeanProxyExt.java:190)
 at $Proxy469.start(Unknown Source)
 at org.jboss.ejb3.JmxKernelAbstraction.install(JmxKernelAbstraction.java:76)
 at org.jboss.ejb3.Ejb3Deployment.startPersistenceUnits(Ejb3Deployment.java:599)
 at org.jboss.ejb3.Ejb3Deployment.start(Ejb3Deployment.java:452)
 at org.jboss.ejb3.Ejb3Module.startService(Ejb3Module.java:139)
 at org.jboss.system.ServiceMBeanSupport.jbossInternalStart(ServiceMBeanSupport.java:289)
 at org.jboss.system.ServiceMBeanSupport.jbossInternalLifecycle(ServiceMBeanSupport.java:245)
 at sun.reflect.GeneratedMethodAccessor220.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
 at java.lang.reflect.Method.invoke(Method.java:585)
 at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:155)
 at org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
 at org.jboss.mx.server.Invocation.invoke(Invocation.java:86)
 at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:260)
 at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
 at org.jboss.system.ServiceController$ServiceProxy.invoke(ServiceController.java:974)
 at $Proxy0.start(Unknown Source)
 at org.jboss.system.ServiceController.start(ServiceController.java:417)
 at sun.reflect.GeneratedMethodAccessor9.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
 at java.lang.reflect.Method.invoke(Method.java:585)
 at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:155)
 at org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
 at org.jboss.mx.server.Invocation.invoke(Invocation.java:86)
 at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:260)
 at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
 at org.jboss.mx.util.MBeanProxyExt.invoke(MBeanProxyExt.java:190)
 at $Proxy29.start(Unknown Source)
 at org.jboss.ejb3.EJB3Deployer.start(EJB3Deployer.java:365)
 at org.jboss.deployment.MainDeployer.start(MainDeployer.java:1007)
 at org.jboss.deployment.MainDeployer.start(MainDeployer.java:997)
 at org.jboss.deployment.MainDeployer.deploy(MainDeployer.java:808)
 at org.jboss.deployment.MainDeployer.deploy(MainDeployer.java:771)
 at sun.reflect.GeneratedMethodAccessor15.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
 at java.lang.reflect.Method.invoke(Method.java:585)
 at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:155)
 at org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
 at org.jboss.mx.interceptor.AbstractInterceptor.invoke(AbstractInterceptor.java:133)
 at org.jboss.mx.server.Invocation.invoke(Invocation.java:88)
 at org.jboss.mx.interceptor.ModelMBeanOperationInterceptor.invoke(ModelMBeanOperationInterceptor.java:142)
 at org.jboss.mx.server.Invocation.invoke(Invocation.java:88)
 at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:260)
 at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
 at org.jboss.mx.util.MBeanProxyExt.invoke(MBeanProxyExt.java:190)
 at $Proxy8.deploy(Unknown Source)
 at org.jboss.deployment.scanner.URLDeploymentScanner.deploy(URLDeploymentScanner.java:334)
 at org.jboss.deployment.scanner.URLDeploymentScanner.scan(URLDeploymentScanner.java:522)
 at org.jboss.deployment.scanner.AbstractDeploymentScanner$ScannerThread.doScan(AbstractDeploymentScanner.java:207)
 at org.jboss.deployment.scanner.AbstractDeploymentScanner$ScannerThread.loop(AbstractDeploymentScanner.java:218)
 at org.jboss.deployment.scanner.AbstractDeploymentScanner$ScannerThread.run(AbstractDeploymentScanner.java:197)
Caused by: java.lang.NullPointerException
 at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1411)
 at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1390)
 at org.hibernate.cfg.Configuration.configure(Configuration.java:1310)
 at org.hibernate.ejb.Ejb3Configuration.createEntityManagerFactory(Ejb3Configuration.java:437)
 at org.hibernate.ejb.Ejb3Configuration.createContainerEntityManagerFactory(Ejb3Configuration.java:245)
 at org.hibernate.ejb.HibernatePersistence.createContainerEntityManagerFactory(HibernatePersistence.java:108)
 at org.jboss.ejb3.entity.PersistenceUnitDeployment.start(PersistenceUnitDeployment.java:260)
 ... 78 more

I guess it has something to do with hibernate 3.1 or the hibernate entity manager that changed from 3.0

I'm using: Jboss-4.0.4RC1 from cvs (today)
postgres 8.1.2
postgresql-8.2dev-501.jdbc3.jar
jdk 1.5.0_6