My guess is that multiclassing as we know it is gone. The class you take at first level determines your defense bonuses, attack bonuses, trained skills, hit points, and so on. If you take the Class Training feat, it gives you the ability to learn powers from a second class, but your total number of powers known in each category (at-will, per-encounter, and per-day) stays the same; you get more powers to choose from, but not more powers known.
That isn't the only possible system, of course, but it's the one that makes the most sense to me. With their emphasis on "the math", anything that allows the class bonuses to stack is problematic, while taking the best bonus would give multiclass characters an advantage over single class characters. That means that the only advantage you'd get from taking a level in another class would be access to different class abilities - but if almost all class abilities are in the form of powers, Class Training gives almost the same result with much less confusion.
In summary: Class Training will be the only multiclass option, but it will give you most of the flexibility of true multiclassing.