Is El Presidente looking at retirement?
"My basic duty is not to cling to office, and even less to obstruct the path of younger people, but to pass on the experiences and ideas whose modest worth stems from the exceptional era in which I have lived," Mr Castro's letter said.
The message was delivered during Cuba's main nightly current affairs programme, Mesa Redonda.
BBC Americas editor Emilio San Pedro says the letter appears to be a calculated attempt to prepare Cuba's 11 million people for a Cuba without the emblematic revolutionary leader in charge.
Independent Cuban journalist Miriam Leiva, whose husband is a former political prisoner, said she believed the announcement might be a turning point for Cuba.
"The situation in Cuba is so tense, economically and socially. This gives the hope that our society can start moving again," she said.
As the BBC report points out, the image of Cuba's power structure as a decrepit Fidel-Raul consulship isn't quite true, and there are relatively young thugs who could take power from Castro. (I wonder if the election-swinging GOP vote in Florida would be quite as hot to turn out if Castro resigned before November 2008.)