class Person < ActiveRecord::Base
  cattr_accessor :current_site_id
  default_scope lambda { where(:site_id => Person.current_site_id) }
end

Person.current_site_id = 1
Person.all
# => SELECT "people".* FROM "people" WHERE ("people"."site_id" = 1)

Person.current_site_id = 2
Person.all
# => SELECT "people".* FROM "people" WHERE ("people"."site_id" = 2)