Really? I count around 16 to 17 states that appear to have very spotty coverage. Hopefully they have good roaming agreements and you don't care about 3G data speeds.
And no one can own an iPhone on ATT in most of those blank areas, because there's only 100% roaming there (no ATT owned towers).
(It's a good bet that people in those large roaming areas who want an iPad 3G also won't be able to sign up for iPad data service. Some reporter needs to check on that.)
By sticking with ATT, Apple is ignoring 2/3 of US cell phone customers on other carriers, and almost 1/2 of US geography that's not natively covered by ATT.
Re: LTE. After early 2014, Verizon will have LTE rolled out to 100% of their current area. No need for CDMA fallback.