Since Congress neglected to exclude horse slaughter from USDA food inspections in its 2012 Agriculture Appropriations Bill, horse slaughter will potentially be legal in states that have not specifically made it illegal starting January 1, 2012. See state status map.
Starting January 1, 2012, there will be eight legal ports of entry for slaughter horses between the United States and Canada. See description and map at http://inspection.gc.ca/english/anima/trans/20111031inde.shtml.
As of Novermber 2011, there are five legal ports of entry for slaughter horses between the United States and Mexico, all on the Texas border. See map at http://www.aphis.usda.gov/vs-tx/impoexpo.htm.
Horse slaughter points of interest, including auction houses, feedlots, border crossings and slaughterhouses, can be found on this Google Map - a work in progress.