def self.authenticate(username, password)
    user = User.find(:first, :conditions => ["username = ?", username])
****    case authenticate_ldap_user(username, password) ****
    when String
      # User authenticated via LDAP.
      if user.blank?
        user = User.new(:username => username, :password_salt => new_salt)
        user.roles << Role.find_by_name('identified')
      end
      user.password = password and user.save! if user.password_hash != encrypted(password, user.password_salt)
      return user if user && user.password_hash == encrypted(password, user.password_salt)
    when FalseClass
      # LDAP raised an exception.
      return false
    when NilClass
      # User not authenticated via LDAP.
      return nil
    end
  end