Rather, the announcement is viewed as a continuation of a transition of power orchestrated by Mr. Castro himself. Analysts expect his brother, Raúl Castro, to be named president of Cuba on Sunday when the party meets to select the State Council and president.
In terms of US-Cuban relations, analysts say, the developments in Cuba fail to satisfy several conditions set by the Bush administration for improved ties. Washington has maintained an economic embargo for 46 years, and President Bush has refused to consider lifting the embargo or otherwise improving relations as long as Fidel Castro, or Raúl Castro, holds the reins of power.
In comments from Rwanda, Mr. Bush repeated his administration's conditions for improved ties to Cuba. "I view this as a period of transition," he told reporters. "It should be the beginning of the democratic transition for the people in Cuba."
Bush said the Cuban government should mark the current transition by releasing political prisoners and by building democratic institutions within Cuba.
John Negroponte, deputy secretary of State, told reporters that Fidel Castro's announcement would not prompt a change in US policy and a lifting of the trade embargo. "I can't imagine that happening any time soon," he said.
Foreign-policy analysts agree. "This will have very little effect on US policy since the Bush administration has made it clear that it won't deal with any Cuban government led by either Fidel or Raúl," says Wayne Smith, director of the Cuba Program at the Center for International Policy in Washington.
"However ... Raúl has indicated he is open to dialogue with the United States; this could lead to some change under a new US administration," he says.
Peter DeShazo, director of the Americas Program at the Center for Strategic and International Studies, says he does not expect any substantial policy change in Washington.
"The administration's policy has been, Cuba should be democratic," Mr. DeShazo says. "The administration has already said, if it's Raúl, or Fidel, it's still the same administration."
But he adds that there are potential openings now. "Raúl Castro has been in de facto control of the country since July 2006, and really controls all the levers of power," DeShazo says. "The real change here is that it gives him still greater legitimacy for making reforms, making change, and I would expect those changes to come in the economic area and not in the political area."
"It's a resignation on paper only," says Ramon Alvarez, a former Cuban government worker who escaped to the US soon after Castro took power. "Many Cubans on the island will be a little emboldened and test the limits a little more, to see if Raúl has the control his brother did. But I don't think anybody believes this will lead to any change in how America treats Cuba, or that anything will improve soon."
Second-generation Cuban-Americans seemed more hopeful over the transition of power in Havana.
"It's a real opportunity for the talking to begin," says Hector Castillo, a construction-firm manager in Miami whose father fled the Castro regime in 1972. "Things might remain the same, and many people will think that Fidel is still in control from the background, but the US cannot know how responsive Raúl will be to change and reform unless it makes the right noises."
Mr. Castillo suggested that it might be a change of government in Washington, rather than Havana, that could make the difference for Cubans. "There are many in my generation who believe the embargo is not working, and despite the problems in Cuba, any meaningful change can only happen if all parties want it," he says. "America has tried to be tough, but the embargo and the travel ban have hurt the people of Cuba, not the leadership. It's time to talk."
On the campaign trail, the two Democratic presidential candidates each put out statements within hours of Castro's announcement. Sen. Barack Obama of Illinois was more explicit in suggesting substantive change to US policy.
"If the Cuban leadership begins opening Cuba to meaningful democratic change, the United States must be prepared to begin taking steps to normalize relations and to ease the embargo of the last five decades," he said.
"As president, I will engage our partners in Latin America and Europe who have a strong stake in seeing a peaceful transition to democracy in Cuba, and who want very much for the United States to play a constructive role to that end," Senator Clinton said.
"We must press the Cuban regime to release all political prisoners unconditionally, to legalize all political parties, labor unions, and free media, and to schedule internationally monitored elections," he said.
"Cuba's transition to democracy is inevitable," Senator McCain also said. "America can and should help hasten the sparking of freedom in Cuba. The Cuban people have waited long enough."
• Richard Luscombe contributed to this report from Miami.