There are no additional details yet on why Jobs is leaving, although the spot assumption is that it's related to the pancreatic cancer for which he received a liver transplant in 2009 (during which time Cook was in charge). The fact that Jobs is taking over as board chairman, rather than resigning that seat too, would seem to indicate that his condition isn't imminently debilitating -- but there also is a strong possibility that the chairmanship is more symbolic than operational.
To the Apple Board of Directors and the Apple Community:
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.
As far as my successor goes, I strongly recommend that we execute our succession plan and name Tim Cook as CEO of Apple.
I believe Apple''s brightest and most innovative days are ahead of it. And I look forward to watching and contributing to its success in a new role.
I have made some of the best friends of my life at Apple, and I thank you all for the many years of being able to work alongside you.