The Microsoft Design team offers developers and designers guidance on the design of app user interfaces on various channels:
DevCenter (Design Section)
Dribble is “a community of designers sharing screenshots of their work, process and projects”. Individual Microsoft designers have been on Dribble before, but now there’s one Microsoft account showcasing all the work in one place.
Following Microsoft Design’s twitter account doesn’t hurt – it is active, but not too much, with a post every couple of days.