This thread from the South America forum has good advice on learning Spanish in South America. I did 2 weeks at a program in Quito, Ecuador and highly recommend it. Subsequently have done 1 week at Nueva Lengua's program in Medellin, Colombia. I love MDE as a vacation destination but the quality of instruction was much better in UIO.
http://www.flyertalk.com/forum/showthread.php?t=503472