Big PC makers such as HP?Compaq and Dell are having a hard time recently as margins get smaller and smaller. Apple, having moved to the Intel processors, are on the verge of losing their uniqueness as well, as their machines are made to run Windows or MacOS natively. It is only a matter of time before MacOS will run on any PC.
Unless Microsoft thinks it can compete in 'coolness' like Apple, they'd just be throwing billions down a rat hole. (See Zune for a perfect example of a failed attempt at Apple-like 'cool.')
For Microsoft, the advantage would be to capture a lot more revenue. It would also allow Microsoft to operate more freely without the constraints of their antitrust conviction.
In theory, it could provide better integration of hardware and software, and perhaps a better quality system overall. But Microsoft has shown little concern for quality so far, so I doubt Microsoft-branded PC would improve the user experience.
If Microsoft is that stupid, I think they just kissed their leading market position byebye. Given how Microsoft has screwed over their partners in the past after introducing a competing product, I would expect the big vendors to bail out of Windows *fast*.