I have a 4 node cluster running on ubuntu precise and, after running well for a few days of testing I now get:

root@nebula02:~# service nova-compute status
nova-compute stop/waiting
root@nebula02:~# service nova-compute start
nova-compute start/running, process 17972
root@nebula02:~# service nova-compute status
nova-compute stop/waiting

2013-10-30 10:07:35.644 17929 INFO nova.virt.driver [-] Loading compute driver 'libvirt.LibvirtDriver'
2013-10-30 10:07:35.648 17929 INFO nova.manager [-] Skipping periodic task _periodic_update_dns because its interval is negative
2013-10-30 10:07:35.696 CRITICAL nova [req-212ae4d9-2206-4e8f-ae0c-4548145093ec None None] 'module' object has no attribute 'packs'
2013-10-30 10:07:35.696 17929 TRACE nova Traceback (most recent call last):
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/bin/nova-compute", line 83, in <module>
2013-10-30 10:07:35.696 17929 TRACE nova     db_allowed=False)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/service.py", line 534, in create
2013-10-30 10:07:35.696 17929 TRACE nova     db_allowed=db_allowed)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/service.py", line 422, in __init__
2013-10-30 10:07:35.696 17929 TRACE nova     self.conductor_api.wait_until_ready(context.get_admin_context())
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/conductor/api.py", line 379, in wait_until_ready
2013-10-30 10:07:35.696 17929 TRACE nova     self.ping(context, '1.21 GigaWatts', timeout=timeout)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/conductor/api.py", line 387, in ping
2013-10-30 10:07:35.696 17929 TRACE nova     return self.conductor_rpcapi.ping(context, arg, timeout)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/conductor/rpcapi.py", line 102, in ping
2013-10-30 10:07:35.696 17929 TRACE nova     return self.call(context, msg, version='1.22', timeout=timeout)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/openstack/common/rpc/proxy.py", line 80, in call
2013-10-30 10:07:35.696 17929 TRACE nova     return rpc.call(context, self._get_topic(topic), msg, timeout)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/openstack/common/rpc/__init__.py", line 140, in call
2013-10-30 10:07:35.696 17929 TRACE nova     return _get_impl().call(CONF, context, topic, msg, timeout)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/openstack/common/rpc/__init__.py", line 301, in _get_impl
2013-10-30 10:07:35.696 17929 TRACE nova     _RPCIMPL = importutils.import_module(CONF.rpc_backend)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/openstack/common/importutils.py", line 58, in import_module
2013-10-30 10:07:35.696 17929 TRACE nova     __import__(import_str)
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/nova/openstack/common/rpc/impl_kombu.py", line 30, in <module>
2013-10-30 10:07:35.696 17929 TRACE nova     import kombu.messaging
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/kombu/messaging.py", line 15, in <module>
2013-10-30 10:07:35.696 17929 TRACE nova     from kombu.serialization import encode
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/kombu/serialization.py", line 330, in <module>
2013-10-30 10:07:35.696 17929 TRACE nova     register_msgpack()
2013-10-30 10:07:35.696 17929 TRACE nova   File "/usr/lib/python2.7/dist-packages/kombu/serialization.py", line 312, in register_msgpack
2013-10-30 10:07:35.696 17929 TRACE nova     registry.register('msgpack', msgpack.packs, msgpack.unpacks,
2013-10-30 10:07:35.696 17929 TRACE nova AttributeError: 'module' object has no attribute 'packs'
2013-10-30 10:07:35.696 17929 TRACE nova

My other compute node doesn't have this problem:
root@nebula03:~# service nova-compute status
nova-compute start/running, process 19145
root@nebula03:~# service nova-compute restart
nova-compute stop/waiting
nova-compute start/running, process 19287
root@nebula03:~# service nova-compute status
nova-compute start/running, process 19287