In order to properly answer this question, we must look into the realms of psychology: The S is a persuasion technique.
Apple just made a ton of money using old hardware, without having to do any real changes, simply by using the principal of parity.
I could take two ordinary Twinkies out of a box, slap an S on one of them, you'd only want the Twinkie S because it sounds better. The same reason they put "new and improved" on the cereal box, even though it's not really new or improved.
Apple could've even just put the 4S's in with the 4's and you'd have been none the wiser.
Speed? Not that you'll notice. It's not the speed that improves, it's the performance, which means less crashes- Yay?
Siri? Apple has already been identified for falsely saying that Siri doesn't work on the iPhone 4 and iPod touch. Could've been in the software update.
Camera? Apple probably, and most assuredly had the 4S planned before the 4 even came out. 720p to 1080p not a breakthrough- predictable.
Apple can't say what the S stands for, because it doesn't stand for anything.