After Apple launched iPhone 4S in October it made a huge success in the worldwide, but Apple first made the iPhone 4S shipping limited for some countries which makes it a little tight, but right now everything has just changed because Apple has just listed iPhone 4S as "In Stock" on all it's online stores:

But Selling 37 million iPhones in Q1 2012, Apple was having trouble keeping up with demand. It’s unclear how many of the 37 million were 4S’s, but it was obviously a majority… So the good news that Apple replaced the origin label “shipping within 24 hours” that you used to see, with the label “in stock” that you now see.