You kind of can already, stick linux on an ssd and just carry it about. I say ssd because not only are they fast, they can be powered over USB.

I suppose the optimal version of this would be to have your userspace carried on a cell phone, so you could plug your phone into any desktop and use it. There's already kind of versions of this, one where the laptop is basically a dumb terminal, and another called drivedroid which allows you to treat your phone as a bootable disk.