While following:

  https://fedoraproject.org/wiki/QA:Testcase_OpenStack_floating_IPs

at around the time that I disassociated or released the floating IP, this error appeared in nova-network's log:

2011-10-20 11:31:00,615 ERROR nova.rpc [-] Exception during message handling
(nova.rpc): TRACE: Traceback (most recent call last):
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/nova/rpc/impl_kombu.py", line 628, in _process_data
(nova.rpc): TRACE:     ctxt.reply(None, None)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/nova/rpc/impl_kombu.py", line 673, in reply
(nova.rpc): TRACE:     msg_reply(self.msg_id, *args, **kwargs)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/nova/rpc/impl_kombu.py", line 781, in msg_reply
(nova.rpc): TRACE:     conn.direct_send(msg_id, msg)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/nova/rpc/impl_kombu.py", line 562, in __exit__
(nova.rpc): TRACE:     self._done()
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/nova/rpc/impl_kombu.py", line 547, in _done
(nova.rpc): TRACE:     self.connection.reset()
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/nova/rpc/impl_kombu.py", line 382, in reset
(nova.rpc): TRACE:     self.channel.close()
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/kombu/transport/pyamqplib.py", line 196, in close
(nova.rpc): TRACE:     super(Channel, self).close()
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/amqplib/client_0_8/channel.py", line 194, in close
(nova.rpc): TRACE:     (20, 41),    # Channel.close_ok
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/amqplib/client_0_8/abstract_channel.py", line 105, in wait
(nova.rpc): TRACE:     return amqp_method(self, args)
(nova.rpc): TRACE:   File "/usr/lib/python2.7/site-packages/amqplib/client_0_8/channel.py", line 273, in _close
(nova.rpc): TRACE:     (class_id, method_id))
(nova.rpc): TRACE: AMQPChannelException: (404, u"NOT_FOUND - no exchange '21937a9d485c483da78f2dbc9aeca78e' in vhost '/'", (60, 40), 'Channel.basic_publish')
(nova.rpc): TRACE: 


Perhaps something to do with em1 device IP changing at that time?