Microsoft is planning a "one-off media event" to show off the new Xbox in early April, based on information from unnamed sources inside and outside of Microsoft. From ArsTechnica:

Practically the entire professional game industry will be gathered together in San Francisco at the end of March for the Game Developers Conference, which would also seem like a natural time for Microsoft to reveal its next-gen plans to an interested audience. Then again, Microsoft could use GDC as a sort of pre-tease tease, letting slip certain small, developer-centric details before a fuller April event.

One thing seems clear: every day people spend talking about the PS4 without any solid information on Microsoft's response is probably being seen as a bit of a missed PR opportunity in Redmond.

 Reports: Microsoft planning to unveil Xbox successor at April event