Apparently, there are at least two recent instances where they have sent out 5 ipads to a customer who has only ordered 1. Their response was to say "keep them." But actually, according to the article and comments made underneath it, the law is that if you are sent items that you didn't order, you can consider them a gift! You have no obligation to return them.
This does present a quandary in some instances. Such as if someone were in jeopardy of losing their job because of the error, or if the mistake were extremely costly to a small company struggling financially. I think only in such circumstances would I go out of my way to return the items.