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I'm not convinced this means his health is failing him. I think he honestly believes it's best for the company as a whole.
Steve is getting older, and he puts absurdly high expectations on himself and those close to him. His tech buddy, Gates, stepped down from MS years and years ago. Tim cook has been fantastic for this company, and as a CEO, could theoretically be better than Jobs himself. Job's strength is his mind, his ability to cut through nonsense and focus on the end goal. His weakness is his social skills, while an unbelievable stage presence, he's apparently a bit abrasive on a personal level.
The job of the CEO is very much to be a figurehead and bring people together. With apple stretching into more and more markets, and being required to interface with more third party companies everyday, Jobs really might not be the best for that position. As the chairman of the board, he can oversee product development and business timelines without being caught up in negotiations with china unicom (for example).
I'm sure this will effect the stock, but I don't see why. Apple still will be selling the same products. The only downsides to this wouldn't be noticeable for years, once apple starts releasing new products not influenced by steve directly. There's no reason to assume from this message that such a day has begun.
Thanks for all the hardwork, Steve. I wish you the best going forward.