I posted this in another thread:
I think there were several factors that went into the decision to put out an upgrade now. It may be that Apple is planning on a reset of its upgrade cycle to coincide wih the holidays. Releasing new equipment in the spring really doesn't tap into the biggest sales season of the year. I'm not sure why anyone would pick the spring season as the time to pump out new products. You lose the excitement of the new release long before the holiday feeding-frenzy starts. Releasing in the fall keeps anticipation and excitement high.
In addition, I think it was important for them to get all their iOS devices on the same docking connector so peripheral vendors have confidence they only have one 'flavor' of connector to plan for and design around. Apple had the same connector for 10 years, so changing to Lightning is a really big deal. They cannot afford to let any of their flagship products languish without the new connector.
Last, I think they needed to make a strategic move to blunt some of the competition in this product category - notably Surface. If anyone has an installed base tied to a specific ecosystem, its Mircosoft. With an installed base of billions of devices, they represent the only real competition to Apple. And they literally own the business ecosystem - from servers to desktops to laptops. Even if they stumble a bit out of the gate, over time they could represent a real threat. And Windows 8 is the only innovative tablet platform to emerge to challenge iOS. The rest are just copy-cats. Apple had to make a move, and they did.
Update - I just thought of something else. All Apple has to do is keep people happy through the 1st and 2nd quarter. By the 3rd quarter the rumor mill is ramping up the excitement for the next product releases and keeping people focused on Apple and their next move. The 4th quarter takes care of itself with the launch events, the holidays and the subsequent demand for the newly updated products.