I think/hope you misunderstand. Where Ballmer really wanted to have one Windows to rule them all, with one crappy UI on all of them, I'm hopeful Nadella is talking more of a unified base with UI adjustments/differences as needed for each device type. You can have a unified release of the base OS with one style interface for tablets, another for desktops, and possibly another for servers. Windows Server has been doing this for a while, with some versions coming with full UI and others with just the CLI. They're a unified release - they come out at the same time and use the same base, but there are different UIs available, similar to one release of Slackware coming with multiple window managers and it being the user's choice which one to use (if any).
So, to give people their "bad car analogy" it's like selling an International DT466 engine in a school bus, a semi tractor, a very large pickup truck, a combine, and a tractor. It's the same engine ("unified release"), but the user picks the chassis/body appropriate for their need. If Microsoft can successfully pull that off, it will be a big win for both the company and consumers.