Microsoft earnings call from earlier today, Nadella laid out the general plans, saying:During the
Nadella didn't give a lot of details, but did say that the engineering team has been unified within Microsoft, so there is now one team building one version of Windows that will be for phones, tablets, PCs, embedded, and even Xbox. He said that the UI will scale across all screen sizes, and he also mentioned the universal app development that has already been released.
Basically, it sounds like Microsoft is planning to converge Windows the same way that Canonical has been working towards with Ubuntu. There will be one OS code base that changes based on the screen size - phone, tablet, PC, and TV - and universal apps that will also adapt to each different screen. Combining this idea with other recent rumors, it sounds like the Windows Phone and tablet UI will stick with the Metro look, while PCs will go with the traditional desktop. We'll surely find out more before the rumored announcement in April 2015.