Yes, you read it right. CNN sends me a breaking news e-mail.
(CNN) -- Fidel Castro announced his resignation as president of Cuba and commander-in-chief of Cuba's military on Tuesday, according to a letter published in the state-run newspaper, Granma.
A historic day indeed.

But Raul Castro replaced his brother. We'll have to wait to see how this all pans out for the Cuban people.
pgl it is not clear that Raul will continue in that role. If I had to guess based on nothing but incomplete data and hubris on my part I would think a likely outcome would be Raul retaining command over the Army and a lead position in the Party while letting the Presidency transition to a slightly younger generation. As a pattern think how the Chinese have handled their succession since Deng, the purely political offices get handed off first with the new guy assuming eventual control of the real levers of power, the PRA and the Party later.
Castro was and to some extent is a ruthless dictator, then again Batista wasn't exactly a saint. I don't think it is wise to fall into the trap of equating Castro to say a Stalin. That is while the Czech Foreign minister may be content to say that the numbers of political prisoners in Cuba is 'enormous'
http://www.iht.com/articles/ap/2008/02/01/europe/EU-GEN-Czech-Cuba-Opposition.php
the article itself goes on to put the number at 250. It is kind of an odd thing that the United States, beacon of liberty, freedom, and human rights to the world, may actually have more political prisoners on Cuban soil than the Cuban dictator does.
And while majorities of that 250 may just be prisoners of conscience, at least some of them have to be guilty of violent plots against the state, it is not like the Exile community in Florida hasn't had elements plotting that all along. But either way even in a small country 250 prisoners is not exactly the Gulag Archipelago.
I'd be willing to assert one thing. Whatever happens, the transition is probably going to be a lot different than if, say, he had died or been overthrown 20 or 30 (or more) years ago. And while I'm not confident enough to predict whether this will lead to any meaningful thaw in US-Cuban relations or speculate on what happens next, I would assess the probability that this will lead to a Pareto improvement (at least for the average Cuban and average American) as marginally greater than it was years ago.
The wild-card is Hugo Chavez. Depending on how close he gets to the new regime, I might have to reassess my probabilities.