Capfile

require 'right_aws'
def instances_from_ec2(load_balancer = nil)
  load_balancer ||= aws_load_balancer
  payload = elb.describe_load_balancers(load_balancer)
  instances = ec2.describe_instances payload.first[:instances]
  instances.collect{|instance| instance[:dns_name] if (instance[:aws_state] == 'running')}
end

def elb
  @elb ||= RightAws::ElbInterface.new(aws_access_key_id, aws_secret_access_key, :region => 'us-west-1')
end

def ec2
  @ec2 ||= RightAws::Ec2.new(aws_access_key_id, aws_secret_access_key, :region => 'us-west-1')
end

production.rb

role :web do
  instances_from_ec2
end