There's a good article in the current New Yorker about the current situation in Cuba. Castro is 80 years old, and successive American governments have had 47 years to perfect plans for once again putting Cuba in the American sphere of influence. Given that GWB is, to our great misfortune, President for two and a half more years, it is quite possible that Castro will be no more and that those plans will be put into place.
What's even more important, I'm not sure that any Democrat would have enough spine to ditch those plans. Let's look at a possible scenario:
Castro dies. American plans for domination swing into action. Hugo Chavez vows to protect the island's sovereignty, and readies his new fighter jets for just that purpose. He threatens to shoot down any American recon planes. Some fishy things start happening in Havana, like riots for "democracy". Tourists are in danger. We send some recon planes. Venezuelan forces shoot them down. We warn Chavez to return all planes to Venezuela. Chavez uses his populist rhetoric to get all of South America to back him.
Argentina sees an opening. They are still trying to get back the Falkland Islands (their Malvinas.) Why not ally themselves with an anti-imperialist stance? There are demonstrations all over the continent against American meddling. But the Cuban-American lobby will not allow any backing down. We install an unstable government in Cuba, Venezuela has all the support it needs, and becomes the Iran of South America. After all, what the US didn't expect was that Chavez was not looking to protect Cuba at all: he just wanted to unite South America in hatred of America.
How could he not succeed? Didn't al-Qaida do the same? Hezbollah?