Microsoft essentially has that with Continuum (not that too many people have Windows Phone). But personally I don't see the value in it. If you only have your phone with you, where are these keyboards and monitors set up (with the cabling required) to use your phone with them? And if you use your phone as the "brains" for your PC in daily use, what if you want to take a call in another room? You have to essentially dismantle your computer?
I think phones and other computers should remain separate and just have better and better synchronization and access to the same data. Apple is at least trying with Continuity & Handoff & stuff like clipboard sharing (when it works!). I'm sure there is some appeal at having only one device, but it hardly even seems like you'd be saving money over having some cheap "phone" hardware embedded into something PC-sized (monitor/keyboard/mouse etc).
Maybe in the far future when everything is completely zero latency wireless, and there is ubiquitous gigabit speed wireless data, and we reach the point where a phone sized device is no real constraint on CPU or GPU capability...
