I’d say write things right now using Famo.us 0.3.5 for practice, but be prepared to rewrite how you build and connect nodes/surfaces. If you write your own view classes, handling Famo.us only in the base classes, then you can write a full fledged app right now, and when Mixed Mode is out you’ll only have to modify the base classes (how things get rendered).
For example, you might have a class called Cube that contains a instances of Square, and finally Square might contain a Surface. When Mixed Mode is out, most of the work would be replacing Square’s Surface with the new Surface.
Or, for example, you might have a base class called “BaseComponent” that contains a RenderNode, a Surface, and an EventHandler, and wires those up, then other classes can do things with the base class (i.e. a Cube would use 6 of them). Then, you only need to modify BaseComponent when MixedMode is out.
Basically, abstract the construction of your component model so when the rendering engine changes you don’t have to change your entire API.