It will be called Office 2013. According to Marry-Jo Folley, TechEd 2012 had a few demos using Office 2013 branding so while this has not been officially announced yet, it would appear to be the case. See the yellow bar in the graphic below
Office 15 = Office 2013
She also says:
Office 2013 is thought to be on a track to RTM in November 2012 and to be generally available in early 2013.