Nobody has even bothered to mention that an important part of 'desktop' systems is the architecture. Desktop systems tend to be able to run on far more horsepower with far more addressable memory space and they tend to be expandable and based on the idea of components (i.e. the screen isn't part of the system, the hard drive is replaceable, memory is often upgradeable and there are video card slots and expansion card slots and a plethora of ports). A "desktop" and "laptop" computer is still considered the workhorse of micro computing whereas cellphones and tablets are mainly based around portable communications and lightweight computing tasks (i.e. mobile apps versus full blown desktop applications, USB for peripherals if you're lucky, etc).

No, again... NOBODY is going to mistake cellphones and tablets for desktops. It's unlikely they'll ever be mistaken for desktops--maybe netbooks/notepads/laptops, but certainly not desktops so long as they're designed around mobility and lack the architecture that desktop have.