Let's break this story down into three sections: Implications for T-Mobile and Apple and then a rerun of the math to see if you really get a better deal by ditching the big three.
T-MobileTo be clear, other than adding iPhone to its stable, this is nothing new for T-Mobile. It's basically looping the iPhone into the mix with the smartphones it already offers. As I'll detail in the math section, that's merely a "no-contract," pay-almost-full-price-for-your-device installment plan. Absolutely excellent marketing, but probably less of a game changer than I initially thought. But that doesn't mean this isn't a big deal.
AppleThis shakes up the industry in a significant way. Apple's acceptance of T-Mobile's installment plan signals yet another shift in the way the company does business. I have to think that before Apple signs off on giving T-Mobile iPhone, it approves how it will be presented and sold to the consumer. Make no mistake about it -- alongside existing deals on iPhone 4 and 4S -- this is the 'cheap iPhone' we have been hearing rumors about. Or at least it better be. Because a "cheap iPhone" would be absolutely awful news for Apple. So if Apple is going to go "cheap," it might as well do it in a way that will not impact its margins much, if at all. I presume T-Mobile pays Apple the same way every other carrier does, except the "un-carrier" will make back that upfront cost direct from the installment plan, not via monthly usage charges.
This represents a compromise by Apple, but it definitely will not dilute its brand image like offering a $100 or $200 phone made of plastic would.