Metadata service sometimes fails to answer

Running with current trunk, starting an Ubuntu image, when cloud-init queries the metadata service requests sometime fail (about 2/3 of the time), with the following error:

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/eventlet/wsgi.py", line 336, in handle_one_response
    result = self.application(self.environ, start_response)
  File "/usr/lib/pymodules/python2.6/paste/urlmap.py", line 203, in __call__
    return app(environ, start_response)
  File "/usr/lib/pymodules/python2.6/webob/dec.py", line 147, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/usr/lib/pymodules/python2.6/webob/dec.py", line 208, in call_func
    return self.func(req, *args, **kwargs)
  File "/usr/lib/pymodules/python2.6/nova/api/ec2/__init__.py", line 59, in __call__
    rv = req.get_response(self.application)
  File "/usr/lib/pymodules/python2.6/webob/request.py", line 919, in get_response
    application, catch_exc_info=False)
  File "/usr/lib/pymodules/python2.6/webob/request.py", line 887, in call_application
    app_iter = application(self.environ, start_response)
  File "/usr/lib/pymodules/python2.6/webob/dec.py", line 147, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/usr/lib/pymodules/python2.6/webob/dec.py", line 208, in call_func
    return self.func(req, *args, **kwargs)
  File "/usr/lib/pymodules/python2.6/nova/api/ec2/metadatarequesthandler.py", line 74, in __call__
    meta_data = cc.get_metadata(remote_address)
  File "/usr/lib/pymodules/python2.6/nova/api/ec2/cloud.py", line 173, in get_metadata
    'instance-type': instance_ref['instance_type'],
  File "/usr/lib/pymodules/python2.6/nova/db/sqlalchemy/models.py", line 74, in __getitem__
    return getattr(self, key)
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/orm/attributes.py", line 163, in __get__
    instance_dict(instance))
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/orm/attributes.py", line 382, in get
    value = callable_(passive=passive)
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/orm/strategies.py", line 578, in __call__
    (mapperutil.state_str(state), self.key)
DetachedInstanceError: Parent instance <Instance at 0x41039d0> is not bound to a Session; lazy load operation of attribute 'instance_type' cannot proceed