We know about 343 and the amazing talent that works their but it seems Microsoft is also picking up talents for their first party games.
Sten Huebler (lead game designer of Crysis 2) is currently working at Microsoft Vancouver. Rumors are the game he is working on is a Space Horror FPS.
Alex Liu (Game Designer at Naughty Dog and worked on Uncharted 3 and The Last of Us. He did the Rough Seas and Caravan mission in UC3) left for Microsoft in May
Also news has come out that Bioshock Infinite Combat Design Director and A.I. Lead has left Irrational for Microsoft this month
Seems like Microsoft is gearing up to launch their next gen consoles with first party support.