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
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.