Via the excellent blog Diverging Markets and the always informative seguridad y justicia page, we get this chart made from stats supplied by Consejo Ciudadano para la Seguridad Pública y la Justicia Penal.
Click chart to significantly ingranderate image (or embiggenfy, if you prefer)
And thusly San Pedro Sula was the most dangerous city in the world last year 2011, beating out Ciudad Juarez and all the others listed. Also note that of the top 20, 100% are in Latin America. Diverging Markets adds the datapoint that of the full 50, 14 are in Brazil, 12 in Mexico and 5 in Colombia.