Actually, no. The chart will officially be dated 2nd January 2010, so Joe would be the first Number One of the 2010s. The Guinness Hit Singles book always lists the Christmas chart as the last chart of the year. :)
(For example, Iron Maiden's Bring Your Daughter to the Slaughter is the first Number One of 1991, despite being announced at the top in December 1990; Chef's Chocolate Salty Balls is the first Number One of 1999, despite being announced at the the top in December 1998 etc...)