Hi John, 
Here're my steps to reproduce:

$ cat devstack/localrc
ENABLED_SERVICES+=,-n-vol,cinder,c-api,c-sch,c-vol
VOLUME_GROUP=stack-volumes
CINDER_BRANCH=folsom-2
NOVA_BRANCH=folsom-2
GLANCE_BRANCH=folsom-2
KEYSTONE_BRANCH=folsom-2
HORIZON_BRANCH=folsom-2

$ nova boot vm1 ...
$ nova volume-create --display_name volume1 1

- vm1 and volume1 are up and active/available
- add manually record to cinder.volume_metadata linked to volume1 with (key=somekey, value=somevalue)

$ nova volume-attach vm1 volume1 /dev/vdc
ERROR: The server has either erred or is incapable of performing the requested operation. (HTTP 500) (Request-ID: req-d4690e16-fcfd-49e6-9f0b-6e4491ebf347)

n-api logs show traceback
2012-07-10 21:04:36 ERROR nova.api.openstack [req-d4690e16-fcfd-49e6-9f0b-6e4491ebf347 admin admin] Caught error: too many values to unpack
2012-07-10 21:04:36 TRACE nova.api.openstack Traceback (most recent call last):
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/api/openstack/__init__.py", line 41, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     return req.get_response(self.application)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/request.py", line 1053, in get_response
2012-07-10 21:04:36 TRACE nova.api.openstack     application, catch_exc_info=False)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/request.py", line 1022, in call_application
2012-07-10 21:04:36 TRACE nova.api.openstack     app_iter = application(self.environ, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 159, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     return resp(environ, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/keystone/keystone/middleware/auth_token.py", line 185, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     return self.app(env, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 159, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     return resp(environ, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 159, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     return resp(environ, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 159, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     return resp(environ, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/pymodules/python2.7/routes/middleware.py", line 131, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     response = self.app(environ, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 159, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     return resp(environ, start_response)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 147, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     resp = self.call_func(req, *args, **self.kwargs)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/usr/lib/python2.7/dist-packages/webob/dec.py", line 208, in call_func
2012-07-10 21:04:36 TRACE nova.api.openstack     return self.func(req, *args, **kwargs)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/api/openstack/wsgi.py", line 849, in __call__
2012-07-10 21:04:36 TRACE nova.api.openstack     content_type, body, accept)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/api/openstack/wsgi.py", line 897, in _process_stack
2012-07-10 21:04:36 TRACE nova.api.openstack     action_result = self.dispatch(meth, request, action_args)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/api/openstack/wsgi.py", line 972, in dispatch
2012-07-10 21:04:36 TRACE nova.api.openstack     return method(req=request, **action_args)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/api/openstack/compute/contrib/volumes.py", line 349, in create
2012-07-10 21:04:36 TRACE nova.api.openstack     volume_id, device)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/compute/api.py", line 104, in wrapped
2012-07-10 21:04:36 TRACE nova.api.openstack     return func(self, context, target, *args, **kwargs)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/compute/api.py", line 1563, in attach_volume
2012-07-10 21:04:36 TRACE nova.api.openstack     volume = self.volume_api.get(context, volume_id)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/volume/cinder.py", line 124, in get
2012-07-10 21:04:36 TRACE nova.api.openstack     return _untranslate_volume_summary_view(context, item)
2012-07-10 21:04:36 TRACE nova.api.openstack   File "/opt/stack/nova/nova/volume/cinder.py", line 92, in _untranslate_volume_summary_view
2012-07-10 21:04:36 TRACE nova.api.openstack     for k, v in vol.metadata:
2012-07-10 21:04:36 TRACE nova.api.openstack ValueError: too many values to unpack
2012-07-10 21:04:36 TRACE nova.api.openstack 

Where vol.metadata is a dictionary, not a list of tuples. 
vol.metadata.items() works for me

Sorry for incomplete reproducing info