I'm pretty sure Apple knows that regardless of the merger outcome, they will get their subsidy payment up front and that any integration period between AT&T and T-Mobile USA would probably take 5-10 years before the former T-Mobile USA drops 3G data service support from their AWS band.I'm pretty sure Apple have been reluctant as of late to release an iPhone with T-Mobile because the future of that carrier is uncertain.
If Apple released the iPhone on T-Mobile USA tomorrow and AT&T acquired T-Mobile USA two weeks from now, it's not like those new T-Mobile iPhone users would be carrying a brick a month from now. Look at the Sprint-Nextel merger; it took many, many years for the service integration.