Posada would be easy enough for the United States to find. In May 2005, after being interviewed in Miami area newspapers, Posada was arrested for entering the United States illegally. He was charged with seven counts of immigration fraud. An immigration judge ruled that Posada could not be extradited to either Venezuela or Cuba because he may face torture there.
Posada was held in a jail in New Mexico until bail was raised. Posada was then on house arrest at his home in Miami. Venezuelan and Cuban officials were furious that the United States would hold a man they perceive to be a dangerous international terrorist on relatively light charges, and allow him to lounge around his house, waiting for trial. The Cuban government states the White House "made all the efforts necessary to protect the bin Laden of the hemisphere, [out of] fear that he could have talked and recount the whole history about the U.S. government links with his terrorists' activities."
I don't remember what happened to Jack Nicholson and his charges of horse stealing. Dr. Alvarez-Machain spent 2 years in federal custody. In December 1992, he was acquitted by a trial judge of having any involvement in the murder of DEA Special Agent Enrique Camarena-Salazar. The court slammed the government's case, stating it was based on "hunches" and the "wildest speculation." Alvarez-Machain returned to Mexico, where he promptly sued the U.S. government and his abductors. Dr. Alvarez-Machain was ultimately awarded $25,000. In May, 2007, the seven counts of immigration fraud against Luis Posada-Carriles were dropped. The judge found procedural mistakes so egregious that she felt "the government's tactics in this case are so grossly shocking and so outrageous [they] violate the universal sense of justice." Luis Posada Carriles is a free man, last seen walking the streets in Miami.
As international terrorism grows and technology shrinks the world, international fugitives will become a larger and larger issue for American law enforcement. We certainly cannot live in a country where once someone crosses a border, they are free from prosecution. But if we are to expect cooperation from the international community, we must be prepared to cooperate. Midnight kidnapping forays into other sovereign nations is no way to run a justice system. But we cannot be hypercritical and do it to other countries if we do not want it done to us. The international law enforcement community should look back on the failures (and successes) of the past and take positive action to ensure there is a reasonable apparatus in place to secure justice for fugitives in foreign countries. Otherwise, those aren't countries; they're just lines on a map.