I think the drink-cart-as-battering-ram is pretty well known to anyone thinking of breaching the cockpit. But now that you mention it, I remember that I was surprised that that aeronautical engineer guy actually admitted something to the effect that "if you apply enough force, the door may fail". (sorry, I don't remember any of the exact wording) I thought he'd say something more discouraging, like you'd have to drive a small car down that narrow plane aisle to have any chance of breaching the reinforced door. Obviously false, but maybe it'll discourage some loony who watches the show.

I sure hope that a real door is strong enough to take more than just two rams from a drink cart.