Sonic for 360 sold 102,250 copies in December. This is well more than 2x November sales. However, I think this game may have been bought more as a gift and so this is not indicative of actual gamer demand. Sonic is now at 141,750 copies sold for North America.
3
http://www.vgcharts.org/usagamemh.php?name=Sonic+The+Hedgehog