Nothing prevents them from changing terms of any plan at any time. They can and most likely will change it at some time, but it could improve just as easily as get worse.....although, given corporate greed, don't count on improvements without some serious competitive reasons for doing so.
I suspect, however, that Apple has likely struck a deal with AT&T to keep the data plan 'really unlimited' for some unspecified period of time, either via a revenue sharing model, exclusivity or some other business agreement. With any luck, you'll see this carried over to other Apple products as well.
But historically, every single 'unlimited' plan in existence has become capped at some time. Given that Apple can reinforce 'soft caps' by forcing app vendors to build in bandwidth-conserving versions of their video/audio apps, this one may have a chance of staying REALLY unlimited.
