I don\'t understand what exactly is going on with this group, and what bundler is doing with it. Is it only loaded in dev mode? What if I want to make a new environment type
The code that handles :assets group placed in config\application.rb. In rails 3.1 it is:
if defined?(Bundler)
# If you precompile assets before deploying to production, use this line
Bundler.require *Rails.groups(:assets => %w(development test))
# If you want your assets lazily compiled in production, use this line
# Bundler.require(:default, :assets, Rails.env)
end