Surface Pro comes close to this - it's very small and easy to carry around, and you could use the Surface docking stations for easier transitions I suppose. The only difference from your dream is that the display doesn't come off, but sometimes it can be an advantage.

What I want is to have docking stations with extra computing power/GPU/RAM/HDD etc built in, so that I can keep the small Surface with its limited battery for the road but have an absurdly powerful desktop PC when I dock it.