After initializing Neutron database and starting neutron-server daemon, Horizon no longer works and the httpd/error_log seems to point to Neutron as the culprit.
The following is logged during a timed-out admin login to the Horizon web interface now:
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/django/core/handlers/base.py", line 109, in get_response
[Wed Jul 16 16:37:21 2014] [error] response = callback(request, *callback_args, **callback_kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/decorators.py", line 38, in dec
[Wed Jul 16 16:37:21 2014] [error] return view_func(request, *args, **kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/decorators.py", line 86, in dec
[Wed Jul 16 16:37:21 2014] [error] return view_func(request, *args, **kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/decorators.py", line 54, in dec
[Wed Jul 16 16:37:21 2014] [error] return view_func(request, *args, **kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/decorators.py", line 38, in dec
[Wed Jul 16 16:37:21 2014] [error] return view_func(request, *args, **kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/decorators.py", line 86, in dec
[Wed Jul 16 16:37:21 2014] [error] return view_func(request, *args, **kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/django/views/generic/base.py", line 48, in view
[Wed Jul 16 16:37:21 2014] [error] return self.dispatch(request, *args, **kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/django/views/generic/base.py", line 69, in dispatch
[Wed Jul 16 16:37:21 2014] [error] return handler(request, *args, **kwargs)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/tables/views.py", line 154, in get
[Wed Jul 16 16:37:21 2014] [error] handled = self.construct_tables()
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/tables/views.py", line 145, in construct_tables
[Wed Jul 16 16:37:21 2014] [error] handled = self.handle_table(table)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/tables/views.py", line 118, in handle_table
[Wed Jul 16 16:37:21 2014] [error] data = self._get_data_dict()
[Wed Jul 16 16:37:21 2014] [error] File "/usr/lib/python2.6/site-packages/horizon/tables/views.py", line 181, in _get_data_dict
[Wed Jul 16 16:37:21 2014] [error] self._data = {self.table_class._meta.name: self.get_data()}
[Wed Jul 16 16:37:21 2014] [error] File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/dashboards/admin/overview/views.py", line 60, in get_data
[Wed Jul 16 16:37:21 2014] [error] data = super(GlobalOverview, self).get_data()
[Wed Jul 16 16:37:21 2014] [error] File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/usage/views.py", line 44, in get_data
[Wed Jul 16 16:37:21 2014] [error] self.usage.get_limits()
[Wed Jul 16 16:37:21 2014] [error] File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/usage/base.py", line 193, in get_limits
[Wed Jul 16 16:37:21 2014] [error] self.get_neutron_limits()
[Wed Jul 16 16:37:21 2014] [error] File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/usage/base.py", line 151, in get_neutron_limits
[Wed Jul 16 16:37:21 2014] [error] api.neutron.is_security_group_extension_supported(self.request)
[Wed Jul 16 16:37:21 2014] [error] File "/usr/share/openstack-dashboard/openstack_dashboard/wsgi/../../openstack_dashboard/api/neutron.py", line 861, in is_security_group_extension_supported