public class Test {
    public static void main(String[] args) {
        Persistence.createEntityManagerFactory("MyPersistenceUnit");
    }
}

run-single:
2010-4-2 0:35:05 javax.persistence.spi.PersistenceProviderResolverHolder$DefaultPersistenceProviderResolver log�
WARNING: javax.persistence.spi::No valid providers found using:�
2010-4-2 0:35:05 javax.persistence.spi.PersistenceProviderResolverHolder$DefaultPersistenceProviderResolver log�
WARNING: javax.persistence.spi::潲朮散汩灳攮灥牳楳瑥湣攮橰愮健牳楳瑥湣敐牯癩摥�
 - jar:file:/D:/NetBeans/java3/modules/ext/eclipselink/eclipselink-2.0.0.jar!/META-INF/services/javax.persistence.spi.PersistenceProvider�
2010-4-2 0:35:05 javax.persistence.spi.PersistenceProviderResolverHolder$DefaultPersistenceProviderResolver log�
WARNING: javax.persistence.spi::潲朮散汩灳攮灥牳楳瑥湣攮橰愮健牳楳瑥湣敐牯癩摥�
 - jar:file:/D:/Java/project%20workspace/Identity/dist/lib/eclipselink-2.0.0.jar!/META-INF/services/javax.persistence.spi.PersistenceProvider�
2010-4-2 0:35:05 javax.persistence.spi.PersistenceProviderResolverHolder$DefaultPersistenceProviderResolver log�
WARNING: javax.persistence.spi::潲朮散汩灳攮灥牳楳瑥湣攮橰愮健牳楳瑥湣敐牯癩摥�
 - jar:file:/D:/Java/project%20workspace/Utilities/dist/lib/eclipselink-2.0.0.jar!/META-INF/services/javax.persistence.spi.PersistenceProvider�
Exception in thread "main" javax.persistence.PersistenceException: No Persistence provider for EntityManager named MyPersistenceUnit�
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

<?xml version="1.0" encoding="utf-8"?>
<persistence xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
  version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
  <persistence-unit name="MyPersistenceUnit" transaction-type="RESOURCE_LOCAL">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <exclude-unlisted-classes>false</exclude-unlisted-classes>
  </persistence-unit>
</persistence>

void test(){

    MainClass.main(null);

}