Here's how it works:
Last week, Apple retail stores started carrying unlocked iPhone 4Ss. However, they only carry enough in stock to sell for that day. So you have to reserve one the night before to guarantee one will be available.
On Apple's website, there is a page for each specific retail location where you can reserve an iPhone 4S (I believe you can only reserve no more than two though). This page is updated at 9pm daily. Be quick to reserve because they can become unavailable very quickly!
After reserving an iPhone 4S, you have until 6pm the following day to pick it up. If you don't pick it up by 6pm, it becomes available for the public.
That is why you may or may not be able to buy an iPhone 4S depending on the time and the number of reserved iPhones.
-SGD