Apple has updated its “how to buy” page for the iPhone 4S to reflect the fact that its retail stores in the United States and Canada are now accepting reservations for the new handset. As stated on the page, “iPhone 4S is available in store by reservation only. Reserve yours after 9:00 p.m. tonight for pickup tomorrow. When you come in, you’ll choose a carrier and plan, and we’ll get your iPhone up and running before you leave the store.” It seems unlikely that Apple would refuse to sell any unreserved units to walk-in customers, and as such, it may be taking reservations and then selling any remaining stock on a walk-in basis. According to an Apple retail employee who spoke with Mac Rumors, their particular location was still selling the iPhone 4S to walk-in customers, adding that supplies are “evaporating.” Apple launched the iPhone 4S on Friday, and this morning announced sales of over four million units over the weekend.