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?