You're wasting your money if you're waiting two years. By that point your phone should be pretty much paid off and you should sell it, especially if you have an iPhone.
The best analogy I can put cellphones into now is leasing a car. With T-Mobile's jump, and 0 down, you get the most bang for your buck if you swap phones every 6 months when you're allowed to.
That's fine, but most people don't actually do that, they go but a new phone when their contract is up. My point was that switching from the 2 year contract model to 24 month financing is not going to change that behavior or significantly impact new phone sales.