In blackpearl there is a subtle distinction:  blackpearl uses WF, but it is not based on WF.  WF schedules are created that contain units of logic but the workflow engine is all K2. 

I do not have insight into the architecture of future products but I would think it would be unlikely this paradigm would change.  And to answer the implied question, yes, you will be hearing more from K2 about .NET 4.0 in the future.