I got an email over the weekend asking about this, and I realized that it’s somewhat buried in this post here. Anyway, in Beta1, you often saw System.Activities.Design. For beta2 (and RTM), one important change
System.Activities.Design => System.Activities.Presentation
The primary reason for this change is that the *.Design suffix is generally reserved for VS design extensibility. As our designer ships in the framework, *.Design was not the correct suffix. *.Presentation is where we landed.
Hoping that putting this in the title lands this high up in search queries so that this post might be useful for a few people.