DefaultLoadEventListener#onLoad has this code:

  if ( idClass != null && ! idClass.isInstance( event.getEntityId() ) ) {
    throw new TypeMismatchException(
      "Provided id of the wrong type. Expected: " + idClass + ", got " + event.getEntityId().getClass());
  }

However, EntityIdentityInsertAction has this in it's constructor:

  delayedEntityKey = isDelayed ? generateDelayedEntityKey() : null;

and method:

  private synchronized EntityKey generateDelayedEntityKey() {
    if ( !isDelayed ) {
      throw new AssertionFailure( "cannot request delayed entity-key for non-delayed post-insert-id generation" );
    }
    return new EntityKey( new DelayedPostInsertIdentifier(), getPersister(), getSession().getEntityMode() );
  }

In case an insert is tried outside of an existing transaction users may run into this problem (like I did).

I don't know what the best fix is. The easiest fix would be:

Index: /Users/eelcohillenius/Documents/workspace/hibernate3/src/org/hibernate/event/def/DefaultLoadEventListener.java
===================================================================
--- /Users/eelcohillenius/Documents/workspace/hibernate3/src/org/hibernate/event/def/DefaultLoadEventListener.java (revision 11098)
+++ /Users/eelcohillenius/Documents/workspace/hibernate3/src/org/hibernate/event/def/DefaultLoadEventListener.java (working copy)
@@ -5,6 +5,7 @@
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.hibernate.EntityMode;
 import org.hibernate.HibernateException;
 import org.hibernate.LockMode;
 import org.hibernate.NonUniqueObjectException;
@@ -10,7 +11,7 @@
 import org.hibernate.NonUniqueObjectException;
 import org.hibernate.PersistentObjectException;
 import org.hibernate.TypeMismatchException;
-import org.hibernate.EntityMode;
+import org.hibernate.action.DelayedPostInsertIdentifier;
 import org.hibernate.cache.CacheConcurrencyStrategy;
 import org.hibernate.cache.CacheKey;
 import org.hibernate.cache.entry.CacheEntry;
@@ -82,7 +83,7 @@
  }
  else {
  Class idClass = persister.getIdentifierType().getReturnedClass();
- if ( idClass != null && ! idClass.isInstance( event.getEntityId() ) ) {
+ if ( idClass != null && ! (idClass.isInstance( event.getEntityId() ) || event.getEntityId() instanceof DelayedPostInsertIdentifier )) {
  throw new TypeMismatchException(
  "Provided id of the wrong type. Expected: " + idClass + ", got " + event.getEntityId().getClass()
  );

but that would look like a quick hack to me.