From eurogamer.net:

The Xbox 360 enjoyed its best ever US sales month in December, according to sales data released by The NPD Group.

Although the DS was the highest selling system of the month, the 360 recorded its best ever figures, shifting 1.9 million units, and was the only console to see a year-on-year sales increase. Impressive figures, considering it ran out of stock before the end of the month.

NPD didn't release information on how the other platforms fared but confirmed that across the board, hardware sales were down six per cent on 2009.
Full Story: eurogamer.net