I don't think this bug is entirely stamped out. I am running a test we created  that heavily stresses the OSAPI and see the following in nova-network.log. I am running

ii  nova-network                2011.3-0ubuntu6.2           OpenStack Compute - Network manager

which should have the change that was supposed to fix this. I verified it is there in the source. This is a single-node configuration of nova. This particular version of the test is creating and destroying vms. I am including the nova.conf at the end.

2011-10-31 10:49:54,178 DEBUG nova.network.manager [51c0fc51-12ab-4a81-9b0b-46b788dbdf63 None None] Leased IP |10.0.0.2| from (pid=104\
10) lease_fixed_ip /usr/lib/python2.7/dist-packages/nova/network/manager.py:645
2011-10-31 10:49:54,314 ERROR nova.exception [-] DB exception wrapped.
(nova.exception): TRACE: Traceback (most recent call last):
(nova.exception): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/exception.py", line 78, in _wrap
(nova.exception): TRACE:     return f(*args, **kwargs)
(nova.exception): TRACE:   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 1400, in flush
(nova.exception): TRACE:     self._flush(objects)
(nova.exception): TRACE:   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 1488, in _flush
(nova.exception): TRACE:     raise
(nova.exception): TRACE: TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType
(nova.exception): TRACE:
2011-10-31 10:49:54,315 ERROR nova.rpc [-] Exception during message handling
(nova.rpc): TRACE: Traceback (most recent call last):
(nova.rpc): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/rpc/impl_kombu.py", line 620, in _process_data
(nova.rpc): TRACE:     rval = node_func(context=ctxt, **node_args)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/network/manager.py", line 655, in lease_fixed_ip
(nova.rpc): TRACE:     'updated_at': now})
(nova.rpc): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/db/api.py", line 402, in fixed_ip_update
(nova.rpc): TRACE:     return IMPL.fixed_ip_update(context, address, values)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/db/sqlalchemy/api.py", line 119, in wrapper
(nova.rpc): TRACE:     return f(*args, **kwargs)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/db/sqlalchemy/api.py", line 885, in fixed_ip_update
(nova.rpc): TRACE:     fixed_ip_ref.save(session=session)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/db/sqlalchemy/models.py", line 59, in save
(nova.rpc): TRACE:     session.flush()
(nova.rpc): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/exception.py", line 81, in _wrap
(nova.rpc): TRACE:     raise DBError(e)
(nova.rpc): TRACE: DBError: exceptions must be old-style classes or derived from BaseException, not NoneType
(nova.rpc): TRACE:
2011-10-31 10:50:01,043 DEBUG nova.rpc [-] received {u'_context_roles': [], u'_msg_id': u'acc9ab22f22f41b39bd453a15d8e3e86', u'_contex\
t_read_deleted': False, u'_context_request_id': u'e63bf738-5258-4383-bcaa-f5330a4ba302', u'args': {u'project_id': u'testproject', u're\
quested_networks': None, u'instance_type_id': 1, u'instance_id': 111, u'host': u'xg11eth0', u'vpn': False}, u'_context_auth_token': No\
ne, u'_context_strategy': u'noauth', u'_context_is_admin': True, u'_context_project_id': u'testproject', u'_context_timestamp': u'2011\
-10-31T14:49:57.960865', u'_context_user_id': u'tester', u'method': u'allocate_for_instance', u'_context_remote_address': u'127.0.0.1'\
} from (pid=10410) __call__ /usr/lib/python2.7/dist-packages/nova/rpc/impl_kombu.py:600

--flagfile=/etc/nova/nova-compute.conf
--use_deprecated_auth
--dhcpbridge_flagfile=/etc/nova/nova.conf
--dhcpbridge=/usr/bin/nova-dhcpbridge
--sql_connection=mysql://nova:notnova@172.18.0.171/nova
--s3_host=172.18.0.171
--rabbit_host=172.18.0.171
--glance_api_servers=172.18.0.171:9292
--logdir=/var/log/nova
--state_path=/var/lib/nova
--lock_path=/var/lock/nova
--verbose
--ec2_url=http://172.18.0.171:8773/services/Cloud
--dmz_cidr=172.18.0.171/32
--fixed_range=10.0.0.0/24
--network_size=256
--image_service=nova.image.glance.GlanceImageService
--bridge_interface=eth1
--flat_network_bridge=br100
--flat_interface=eth1
--network_manager=nova.network.manager.FlatDHCPManager
--public_interface=eth0
--multi_host=1
--osapi_host=172.18.0.171
--quota_instances=1000000
--quota_ram=1000000
--quota_cores=100
--iscsi_ip_prefix=172.18.0