Hi Thomas,

I try to reproduce your situation, and found the same result you provide.
Then I try to replace all FLAGS. with self.configuration. in cinder/volume/drivers/nexenta/volume.py and there are new error messages.

2013-05-21 19:14:41     INFO [cinder.volume.manager] volume volume-c3f86e6a-db2b-441c-8507-0f531c1fcafe: creating
2013-05-21 19:14:41    ERROR [cinder.volume.manager] Error: ['Traceback (most recent call last):\n', '  File "/usr/lib/python2.7/dist-packages/cinder/volume/manager.py", line 251, in create_volume\n    image_location)\n', '  File "/usr/lib/python2.7/dist-packages/cinder/volume/manager.py", line 168, in _create_volume\n    model_update = self.driver.create_volume(volume_ref)\n', '  File "/usr/lib/python2.7/dist-packages/cinder/volume/drivers/nexenta/volume.py", line 123, in create_volume\n    self._get_zvol_name(volume[\'name\']),\n', '  File "/usr/lib/python2.7/dist-packages/cinder/volume/drivers/nexenta/volume.py", line 105, in _get_zvol_name\n    return \'%s/%s\' % (self.configuration.nexenta_volume, volume_name)\n', "NameError: global name 'self' is not defined\n"]
2013-05-21 19:14:41    ERROR [cinder.volume.manager] volume volume-c3f86e6a-db2b-441c-8507-0f531c1fcafe: create failed
2013-05-21 19:14:41    ERROR [cinder.openstack.common.rpc.amqp] Exception during message handling
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/cinder/openstack/common/rpc/amqp.py", line 430, in _process_data
    rval = self.proxy.dispatch(ctxt, version, method, **args)
  File "/usr/lib/python2.7/dist-packages/cinder/openstack/common/rpc/dispatcher.py", line 133, in dispatch
    return getattr(proxyobj, method)(ctxt, **kwargs)
  File "/usr/lib/python2.7/dist-packages/cinder/volume/manager.py", line 282, in create_volume
    LOG.error(_("volume %s: create failed"), volume_ref['name'])
  File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
    self.gen.next()
  File "/usr/lib/python2.7/dist-packages/cinder/volume/manager.py", line 274, in create_volume
    model_update = self.driver.create_export(context, volume_ref)
  File "/usr/lib/python2.7/dist-packages/cinder/volume/drivers/nexenta/volume.py", line 250, in create_export
    loc = self._do_export(_ctx, volume, ensure=False)
  File "/usr/lib/python2.7/dist-packages/cinder/volume/drivers/nexenta/volume.py", line 193, in _do_export
    zvol_name = self._get_zvol_name(volume['name'])
  File "/usr/lib/python2.7/dist-packages/cinder/volume/drivers/nexenta/volume.py", line 105, in _get_zvol_name
    return '%s/%s' % (self.configuration.nexenta_volume, volume_name)
NameError: global name 'self' is not defined

So it seems the function with @staticmethod annotation must use FLAGS to access the configurations.
Since FLAGS.register_opts(nexenta_opts) only gets the default value, I suggest you two temporary solutions:
1.  Use the default Nexenta Volume name (i.e cinder)
2. Change the default Nexenta Volume name used in cinder/volume/drivers/nexenta/volume.py
    cfg.StrOpt('nexenta_volume',
               default='ZPOOL_TEST',
               help='pool on SA that will hold all volumes'),

Because I am not the author of Cinder nor Nexenta Driver, I can not sure what's the problem on FLAGS.
This is my best solutions I can give you.