Interestingly enough, Rowling addresses this in passing in the new book (no spoilers)
Apparently, according to Hermione, there are five exceptions to "Gamp's Law of Elemental Transfiguration," i.e. five things that can't be created out of nothing by magic. Food is one of them. As you recall, the food that appeared at Hogwarts was actually cooked downstairs and magically transported upstairs.
This neatly covers a lot of territory, especially since I don't know the other four items (I suspect precious metals would be one).
Reminds me of some obscure law the Doctor (from Dr. Who) keeps quoting to explain why he can't use his time machine to fix things by going back and not screwing up. The Laws of Time seem to be extraordinarily flexible, and tuned to making the story work more than anything else.