I was really excited that they included the apple, and then I started thinking about what rôle the apple has in the story. Last month, I finally re-read "The Metamorphosis" to see if the apple could represent original sin. The edition of "The Metamorphosis" that I have includes critical essays, which helped a bit in determining whether my view was a valid one (my view certainly isn't as far-fetched as some). I think I've come to the conclusion that the apple could represent original sin, but only narrowly, as other elements of the story don't have the same religious context.
Like original sin is inherited from one's parents after Eve took the fruit from the tree, Gregor is pummeled with apples by his father. My edition notes the "religious connotation of [Kafka's] images," which includes the apple. The explanatory notes in my edition confirm this connection between the apples and the tree (and between the tree and sin), adding that - unlike Eve - Gregor does not reach out for the fruit, but it is forced upon him. The story also mentions debts that the father has that Gregor has to pay. In some ways, this financial debt could be seen as a sort of moral deficiency. The explanatory notes remark that "the debt that Gregor assumed for his parents and must pay resembles original sin." Furthermore, Gregor's metamorphosis into a bug sort of reflects his fallen state. As the introduction in my edition states, as a bug Gregor is "without a place in God's order."
As a whole, I don't think the story works as a religious allegory (if it were meant to be allegorical, I think there would be some salvation for Gregor that would restore him to his human form), but the image of the apple certainly works within that religious context.
