gistfile1.rb

module ActsAsDuck
  
  def self.included(base)
    base.extend ClassMethods
  end
  
  module ClassMethods
    def acts_as_duck(*args)
      options = args.extract_options!
      class_name = options[:name] || self.class.to_s + "Duck"  # <-- problem here:  self.class always is "Class"

      module_eval(<<-EVAL, __FILE__, __LINE__)      
        class #{class_name}
          def quack; puts "quack"; end
        end
      EVAL
  end
end