This is crazy talk. Microsoft needs a Windows compatible OS so they can leverage the millions of windows applications already written. To start from scratch would give up one of their main advantages. Best to stick with Windows, and focus on optimizing Windows 8 and Office 2012 for the tablet form factor. I love my iPad but I'd be willing to take a half-step back to a Windows tablet if it meant I could run Microsoft…
I've used a lot of shareware and small developer type apps on osx, windows and linux, and the osx stuff is noticeable better in the UI department; it's pretty brutal how little effort most small dev's put into the UI or even graphic design.
Even large developers have trouble creating well designed Ui's; I'm thinking of Quicken, which has always struck me as amateurish and dated though I haven't used it in a while so maybe it's improved.
Point being, if you simply put a desktop OS on a tablet, most developers aren't going to do anything to make the experience great. This is something clearly lost on Ballmer as well.