There are reports that the next XBOX will debut at the Electronic Entertainment Expo in May. The company has confirmed that it will not debut at the Game Developers Conference in March, so E3 is the next logical choice. Five years ago, it was at GDC that Microsoft debuted the first XBOX.
According to Engadget, Microsoft has confirmed E3, but I can’t seem to find that anywhere. More likely they just interpreted the GDC-related confirmation as proof that E3 is the target launch vehicle now. Can’t wait to see what it looks like!