Remember that because of the way Apple chooses to do business, they can't change models with the same frequency other manufacturers do. They've been making essentially (with minor changes) the same 3GS model for over 2 yrs. and the iPhone 4 will have at least the same life span. This is the only way they can lock in component pricing and supply for the extremely steep production ramps. They're going to have to go from zero to 10 million per month for this 5th gen iPhone. That takes $billions in long term commitments to pull that off smoothly. Even then, it took them 5-6 months before supply of iPhone 4 met demand. I'm sure Tim Cook wants to shorten the time it takes to meet demand.
Compare that to Samsung with the Galaxy S2. How many different screen sizes and configurations do they build for the various cell carriers? Yet with all those production gyrations, Samsung will be lucky to sell 10 million units over the course of one year. So why would Apple "fix" what is not broken?
New iPad in Mar./Apr. and new iPhone in Sep./Oct. sounds like a devastating iOS 1-2 punch. Sprinkle in new iPods and Macs throughout the rest of the year and you've got the tech press buzzing about Apple pretty much 24/7.