It's no joke. From the official press release:
I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know. Unfortunately, that day has come.
I hereby resign as CEO of Apple. I would like to serve, if the Board sees fit, as Chairman of the Board, director and Apple employee.
While I certainly can't say I've ever seen eye-to-eye with Apple's way of treating its users like badly behaved four-year-olds, I certainly hope that this is not an issue with Steve's health. All my best to him.