The lines will blur anyway. A Microsoft Surface combined with a TypeCover (assuming it works well), could be used as both a tablet and a laptop at different times. Does it then contribute to PC sales or tablet sales?
The important difference is 'internet/media consumption device' (iPad) vs. 'general computing and productivity device' (PC, Surface?). I believe the growth and shrinkage of their respective markets indicate the typical consumer's desires.
Microsoft failed in previous tablet efforts by trying to graft the desktop environment onto a touchscreen. Failed miserably, and not because of hardware immaturity. In contrast, Apple has succeeded immensely by discarding the desktop metaphor and embracing the touch environment exclusively. Microsoft is trying to straddle the two environments solely to prop up Windows, not because it's a better UX. This to me, dooms it to failure, just as Microsoft failed in its previous tablet ventures.