It's a rough estimate based on 2008 total incidence numbers combined with the 2006 homo / hetero and the 2007 male / female split, numbers JAMA / CDC.
http://www.cdc.gov/hiv/topics/surveillance/incidence.htm
http://www.cdc.gov/hiv/topics/surveillance/basic.htm (scroll down to "HIV/AIDS Cases by Transmission Category")
However, the national average is useless in determining one's own personal risk. as the CDC also points out, there is no HIV epidemic in rural Kansas, on the other hand HIV amongst heterosexuals is probably transmitted at a more than sustainable rate in cities like LA, NY, Atlanta or Miami.