The US considered annexing Cuba in the mid-1800s and split it into several states (actually, that was driven by southerners who wanted more slave states.) It didn't happen, and probably won't ever; even if Castro's gone that doesn't mean the Cubans want to become a full-fledged part of the US. It's more likely that the 51st state will be "Saudi Israelia" (a "Simpsons" reference) 
Washington DC is more likely to become a state than Cuba, and DC isn't becoming a state anytime soon. Puerto Rico is somewhat more likely, but not much so (maybe it could be united with the Virgin Islands?) Maybe the Pacific Islands we control (Guam, Wake Island, etc.) if put together could qualify, but such a state would be too far flung to be coherent; imagine trying to run for governor or senator while having to island-hop for thousands of miles.
Maybe if the Canada/Quebec tensions get bad enough, the Quebecois might look south?
Quebec would want independence, not union with the US. I agree that it's highly unlikely Cuba will ever become a state, and if it does, it probably won't vote Republican. Cuban-Americans in Miami vote Republican because of fidel, and with him gone, they'll have no reason to do so. Besides, the average Cuban has very different priorities and ideas, though it's difficult to say what those are due to censorship on the island.
A Pacific state made up of Guam, the Northern Marianas, and American Samoa, plus Palau, the FSM, and the Marshall Islands (which are all currently nominally independent, but so small and dependent on the US that they might benefit from annexation) would be a more realistic possiblity than either of those two.
