Of course humans form a single species. But I would contend they are fairly obviously divided into a number of subspecies, just as many other animal species are.
BTW, the term “race” originally meant something very much along the line of subspecies, and is still used that way in botany and zoology, as in “landrace,” used to describe variants in a single species that have significant variability but are still interfertile.
http://en.wikipedia.org/wiki/Landrace
Maybe we could use the term landrace and confuse the hell out of the “antiracists!”