generic_lookup.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  before_filter :lookup, :only => [:show, :destroy, :edit, :update]
  
  def lookup
    class_name = params[:controller].singularize
    klass = class_name.camelize.constantize
    self.instance_variable_set "@" + class_name, klass.find(params[:id])
  end
end

Interesting, definitely a common pattern.