I will probably get down ranked for this, so have at it.
Apple BETTER have kept the iPhone 5 as one of their biggest secrets ever. If the event comes and goes with a 4S, with an A5 processor and an 8MP camera and some minor tweaks, a few things will happen.
First, Tim Cook may lose some respect. He has been at the helm for the past 9 months, and the iPhone, following Apple's usual schedule, is about 3 months late. If Tim doesn't release a major, innovative product, I fear many investors will think that "Steve Jobs will always be better". In my head, Steve Jobs should have been the best, and he was, while he was the CEO, now it's someone else's turn.
Secondly, I fear Apple will start losing some major market share and fast. The iPhone 4 is a beautiful phone. The screen resolution is amazing. Battery gets me through the day without worries. And it's pretty quick. However, Apple has prided itself on being this major innovative company, when frankly the A4 chip is under clocked to around 833MHz, 512MB of RAM doesn't cut it, and many other critiques. The A5 would add 1GHz, and that's great, however new phones are coming out with 1.2 and 1.5GHz clock speeds, and Apple will have their newest phone on the market running a slower processor. Not very innovative if you ask me. Also, I have no doubt in my mind that if a 4S is released, the screen resolution will stay the same. And while the iPhone 4's screen is great now, in a few months when 720p resolution screen are getting released, Apple will again be behind. I wouldn't care too much, just stop saying you're this big, innovative company. If you want to say that about yourself, then start proving it, because a 4S isn't too much proof.
Thirdly, and this is the most unimportant, I will look elsewhere for my cellular phone needs. I love Apple, I'll never get rid of my Macbook Pro or iPad, frankly I feel they are the best, and most innovative. But honestly, I do not like the iPhone 4 design. I like my phone naked, and while I haven't had any major incidents, keeping the iPhone 4S in pristine condition would just be another year of hell. Not to mention 4G has been "blazing" through my area for about 4 months now, and I would really like to get on that bandwagon.
Apple will do what they feel is best, although I feel their judgement has always been somewhat clouded. They have amazing software and they tie it in perfectly with their hardware, and it is such a great experience. I simply don't want a 4S for myself, and I am not sticking with my iPhone 4 until possibly next October. Others may feel differently, and that's fine. This is what I see happening if the next iPhone is a minor upgrade.