Obviously, it's only the top 3 because the iPhone still isn't available on T-Mobile, but the predictions seem very reasonable. The basic idea is that in Q3, the iPhone 4 and 3Gs were the two available models. The 4 was available on both AT&T and Verizon, while the 3Gs was AT&T only. Those two models accounted for just over 45% of smartphone sales on those two carriers. The iPhone 4 sold 2 million units on Verizon (out of 5.6 million smartphones), and 2.7 million on AT&T (out of 4.8 million). Of course, in Q4 there will be more available iPhone models, and Sprint gets added into the mix. But, on top of that, Q3 was a down quarter for Apple because everyone knew that the new iPhone was coming in October.
So, with just an aging iPhone 4 and a legitimately old iPhone 3Gs in a quarter when people were holding off because of a new release on the horizon, Apple still managed 45% of the smartphone sales on Verizon and AT&T. In Q4, there is not only the new iPhone 4S, but the iPhone 4 is available on the top 3 carriers at a discount, and AT&T users on a very strict budget can still grab an iPhone 3Gs for free. Given that we know the 4S sold 4 million units in its first week alone in worldwide sales, it seems easy to expect this will be a huge quarter for Apple. Some analysts have the numbers much higher, but at least 50% of smartphone sales on Verizon, AT&T and Sprint is definitely within reach.