Had it working once like this

gistfile1.txt

# pakyow-auth.rb

module Pakyow
  module Auth

    def self.included(obj)
      if defined?(DataMapper)
        require 'pakyow-auth/orm/datamapper/user'
        require 'pakyow-auth/orm/datamapper/session'


        # obj.send(:include, DataMapper::Resource)
        # obj.class.extend Pakyow::Auth::UserMethods if obj.name == "User"
      else
        puts "YIKES!! We don't have Datamapper!!!!"
      end
    end
  end
end

user.rb

class User
      include DataMapper::Resource

      class << self
        @@login_field = :email
        
        def login_field
          @@login_field
        end
      end

      attr_accessor :password, :password_confirmation

      storage_names[:default] = "users"
           
      property :id,                   Serial
      property :crypted_password,     String
      property :salt,                 String
      property :password_reset_token, String
      property :password_reset_token_expiration, DateTime

      def create_password_reset_token
        self.password_reset_token = rand(36**64).to_s(36)
        self.password_reset_token_expiration = Date.today + 5
      end

      def password=(p)
        return if p.nil? || p.empty?
        @password = p
        
        self.salt = Digest::SHA1.hexdigest("--#{Time.now.to_s}--#{User.login_field}--") 
        self.crypted_password = encrypt(p)
      end
      
      # Authenticates a user by their login name and unencrypted password.  Returns the user or nil.
      def self.authenticate(session)
        u = self.first(self.login_field => session.login) # need to get the salt
        if u && u.authenticated?(session.password)
          return u
        else
          return false 
        end
      end

      def authenticated?(password)
        true if self.crypted_password == encrypt(password)
      end

      private

      # Encrypts the password with the user salt
      def encrypt(password)
        self.encrypt(password, salt)
      end

      # Encrypts some data with the salt.
      def self.encrypt(password, salt)
        Digest::SHA1.hexdigest("--#{salt}--#{password}--")
      end
    end