From gamasutra.com:

Xbox Live program manager Larry "Major Nelson" Hryb said on Twitter that Microsoft "ran out of consoles" at the end of December, and that as a result those on the Xbox team "don't expect to win [December]."

Nelson warned that Xbox 360 hardware supply for January and February would be "tight as well," but that December's NPD results should bring "likely amazing [year-over-year] growth numbers for Xbox."
Full Story: gamasutra.com