It is something for Microsoft/Google to be aware of, but I don't think it will be something to be worried about as I think it's going in the wrong direction. With a 13" iPad, I agree that they'll be cannibalizing their Macbook Air market... however they may do something like drop the air and have the iPad run iOS/OSX or something like that. The only thing that will happen is the media will praise them for anything they copy and make their own saying what innovators.
The points need to be focused on:
1) In the near future, the convergence of WP8/W8 and iOS/OSX. Google is already converged but doesn't have the software that business wants. What I mean by convergence is that I should be able to buy an app and have it run on both the mobile os and the desktop os for one purchase, have it share the same data, etc...
2) In the not too distant future, think 3 year to 5 year plan, one device to rule them all. The smartphone or a similar device as the core. You carry it as a phone, the interface is geared towards a phone. You want a tablet, you plug it into a tablet case and now you have all the features of a tablet with a tablet interface. You want a laptop, you plug it into a laptop case and now you have the features of a laptop. You want a desktop, you plug it into a desktop case you gain those features.
They could even do something cool, like when you plug it into the desktop case, it will use the graphics card in that case if you have one installed, or a supplemental CPU, or supplemental RAM...
Or we could just have all separate devices that sync over the cloud... as long as I can use an app on my phone, tablet, laptop, and desktop with the same data source I'd be fine.