Elasticsearch - EC2 Region/Availability zone problem

Hi there,
We've got a bit of a problem with our elasticsearch environment on AWS in the US-West-1 region. As far as we can tell, what's happening is that the cloud-aws plugin is treating availability zones as distinct regions, instead of including the AZs we are specifying.

elasticsearch.yml contains the following specific settings:
network.bind_host: _eth0:ipv4_
#cloud.aws.region: us-west-1
cloud.aws.access_key: <DELETED>
cloud.aws.secret_key: <DELETED>
cloud.node.auto_attributes: true
discovery.type: ec2
discovery.any_group: true
discovery.ec2.tag.stage: production
discovery.ec2.availability_zones: us-west-1a,us-west-1c
gateway.type: s3
gateway.s3.bucket: <our-bucket-name>
gateway.s3.concurrent_streams: 5
gateway.expected_nodes: 3
gateway.recover_after_time: 1m
gateway.recover_after_nodes: 1
indices.recovery.max_size_per_sec: 0
indices.recovery.concurrent_streams: 5
cluster.routing.allocation.node_concurrent_recoveries: 2
discovery.zen.ping_timeout: 30s

cloud.aws.region is commented out because when it isn't commented out, starting elasticsearch gives an error as follows and then crashes out:
[2012-01-03 18:42:24,333][WARN ][com.amazonaws.http.AmazonHttpClient] Unable to execute HTTP request: null
[2012-01-03 18:42:24,344][WARN ][com.amazonaws.http.AmazonHttpClient] Unable to execute HTTP request: null
{0.18.6}: Initialization Failed ...
1) AmazonClientException[Unable to execute HTTP request: null]
ProtocolException[Received redirect response HTTP/1.1 301 Moved Permanently but no location header]

With the above custom config, when starting up either node (both in different AZs) we get this in the logs on each:

[2012-01-03 19:06:14,454][INFO ][cluster.service ] [Bulldozer] new_master [Bulldozer][4QTO0FKfQkGiUOgGHncu_w][inet[/10.169.49.245:9300]]{aws_availability_zone=us-west-1a}, reason: zen-disco-join (elected_as_master)

[2012-01-03 19:06:23,301][INFO ][cluster.service ] [Mad Dog Rassitano] new_master [Mad Dog Rassitano][vPKkND4UTJ6o245oHL8efQ][inet[/10.171.49.112:9300]]{aws_availability_zone=us-west-1c}, reason: zen-disco-join (elected_as_master)

We only want one to be a master for the cluster - they simply won't cluster at all.

Can anyone point out what we are doing wrong here?
James.