Let’s face it: iPad subscriptions are the holy grail for magazine publishers hoping to make beaucoup bucks from iPad publications like Project and News Corp’s iPad newspaper, The Daily. Now there’s another good sign that the function will arrive before Christmas. The Guardian has released a statement saying it will kill the current Guardian app and replace it with a subscription based app. Meanwhile US Guardian readers will get an ad-supported app. The Guardian iPad app will emerge before Christmas and the timing fits with an Apple event this month. We are now watching our virtual postbox for that Apple invite…