These are the tracebacks we're seeing when this bug is invoked.

2012-03-26 19:21:28 ERROR nova.compute.manager [-] [instance: 2967f7b5-ae4a-4511-a8b0-10f330b7d5df] Instance failed to spawn
(nova.compute.manager): TRACE: Traceback (most recent call last):
(nova.compute.manager): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 592, in _spawn
(nova.compute.manager): TRACE:     self._legacy_nw_info(network_info), block_device_info)
(nova.compute.manager): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/exception.py", line 114, in wrapped
(nova.compute.manager): TRACE:     return f(*args, **kw)
(nova.compute.manager): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/connection.py", line 926, in spawn
(nova.compute.manager): TRACE:     self.firewall_driver.prepare_instance_filter(instance, network_info)
(nova.compute.manager): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/firewall.py", line 136, in prepare_instance_filter
(nova.compute.manager): TRACE:     self.add_filters_for_instance(instance)
(nova.compute.manager): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/firewall.py", line 178, in add_filters_for_instance
(nova.compute.manager): TRACE:     ipv4_rules, ipv6_rules = self.instance_rules(instance, network_info)
(nova.compute.manager): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/firewall.py", line 334, in instance_rules
(nova.compute.manager): TRACE:     ips.extend(net[1]['ips'])
(nova.compute.manager): TRACE: KeyError: 1
(nova.compute.manager): TRACE:

Then again when trying to deallocate the network for the instance.

2012-03-26 19:21:28 DEBUG nova.compute.manager [-] [instance: 2967f7b5-ae4a-4511-a8b0-10f330b7d5df] Deallocating network for instance from (pid=32203) _deallocate_network /usr/lib/python2.7/dist-packages/nova/compute/manager.py:616
2012-03-26 19:21:28 DEBUG nova.rpc.amqp [-] Making asynchronous cast on network... from (pid=32203) cast /usr/lib/python2.7/dist-packages/nova/rpc/amqp.py:346
2012-03-26 19:21:28 ERROR nova.rpc.amqp [-] Exception during message handling
(nova.rpc.amqp): TRACE: Traceback (most recent call last):
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/rpc/amqp.py", line 252, in _process_data
(nova.rpc.amqp): TRACE:     rval = node_func(context=ctxt, **node_args)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/exception.py", line 114, in wrapped
(nova.rpc.amqp): TRACE:     return f(*args, **kw)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 177, in decorated_function
(nova.rpc.amqp): TRACE:     sys.exc_info())
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
(nova.rpc.amqp): TRACE:     self.gen.next()
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 171, in decorated_function
(nova.rpc.amqp): TRACE:     return function(self, context, instance_uuid, *args, **kwargs)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 648, in run_instance
(nova.rpc.amqp): TRACE:     self._run_instance(context, instance_uuid, **kwargs)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 451, in _run_instance
(nova.rpc.amqp): TRACE:     self._set_instance_error_state(context, instance_uuid)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
(nova.rpc.amqp): TRACE:     self.gen.next()
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 432, in _run_instance
(nova.rpc.amqp): TRACE:     self._deallocate_network(context, instance)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
(nova.rpc.amqp): TRACE:     self.gen.next()
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 429, in _run_instance
(nova.rpc.amqp): TRACE:     injected_files, admin_password)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 592, in _spawn
(nova.rpc.amqp): TRACE:     self._legacy_nw_info(network_info), block_device_info)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/exception.py", line 114, in wrapped
(nova.rpc.amqp): TRACE:     return f(*args, **kw)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/connection.py", line 926, in spawn
(nova.rpc.amqp): TRACE:     self.firewall_driver.prepare_instance_filter(instance, network_info)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/firewall.py", line 136, in prepare_instance_filter
(nova.rpc.amqp): TRACE:     self.add_filters_for_instance(instance)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/firewall.py", line 178, in add_filters_for_instance
(nova.rpc.amqp): TRACE:     ipv4_rules, ipv6_rules = self.instance_rules(instance, network_info)
(nova.rpc.amqp): TRACE:   File "/usr/lib/python2.7/dist-packages/nova/virt/firewall.py", line 334, in instance_rules
(nova.rpc.amqp): TRACE:     ips.extend(net[1]['ips'])
(nova.rpc.amqp): TRACE: KeyError: 1
(nova.rpc.amqp): TRACE: