Revit 2010 released in 2009, Revit 2009 released in 2008 and so on. It boils down to this, Autodesk's fiscal year is currently 2010 so therefore is the software it makes. I guess it is kind-a-sort-a like the car industry, selling a 2009 model in the fall of 2008??
At least it is better than trying to make sense of the Revit compatibility between the previous naming scheme; Revit Building 9.0, Revit Structure 3.0 and Revit Systems 2.