Imported From: http://groups.google.com/group/in-portal-bugs/browse_thread/thread/415fb5543269d755#
Line 790 if ( $object->isRequired('Username') ) {
$object->SetDBField('Username', $user_email);
}
First mentioned line ensures that 'Username' field is never set, and,
as result - subscriber-user record is not created with error "username
is too short" (as fact, it is empty, so it length is 0).