foo.rb

# Install the psych gem and uncomment the following line.
#gem 'psych'
require 'psych'

stuff = Psych.load_file ARGV[0] || 'newrelic.yml'
p stuff['production']['enabled']

newrelic.yml

common: &default_settings
  license_key: 'aee....517'
  app_name: AkitaOnRails.com
  enabled: false
  log_level: info
  ssl: false
  apdex_t: 0.5
  capture_params: false
  transaction_tracer:
    enabled: true
    transaction_threshold: apdex_f
    record_sql: obfuscated
    stack_trace_threshold: 0.500
  error_collector:
    enabled: true
    capture_source: true    
    
development:
  <<: *default_settings
  enabled: false
  developer: true
test:
  <<: *default_settings
  enabled: false
production:
  <<: *default_settings
  enabled: true
staging:
  <<: *default_settings
  enabled: true
  app_name: My Application (Staging)

rails console output

Running Rails 3.0.5 and Ruby 1.9.2-p180 in both OS X (Snow Leopard 10.6.7) and Debian Lenny, different override results for the YAML parsing:

~/Sites/rails/enki ruby-1.9.2(master $) ruby --version
ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.6.0]

> ruby-1.9.2-p180 :004 > a["production"]["enabled"]
 => true 

root@webby4106:/var/webapps/akitaonrails# ruby --version
ruby 1.9.2p180 (2011-02-18 revision 30909) [i686-linux]

> ruby-1.9.2-p180 :002 > a["production"]["enabled"]
 => false