The press release does not go into detail as to the reasons for the leave of absence. Steve Jobs is a very private person, and it is unlikely we will hear further, at least officially. Due to the great influence news about Steve Jobs has on the company's stock, Apple has previously been criticized for a lack of openness with regards to his medical condition.
While Apple fared well the last time Tim Cook held the reins, Jobs always has his hands, for better or worse, in just about every important product the company releases. A lengthy leave of absence, or even a retirement, could be bad news for Apple, and also, at least in the short term, for shareholders.
The announcement came on a stock market holiday, Martin Luther King Day. Any effect on the stock won't truly show until Tuesday.
It's unclear if this leave is related to Jobs' past medical issues. In 2004, Jobs contracted pancreatic cancer, which he overcame. In 2009, he took the previously mentioned leave of absence, and eventually had a liver transplant.
The company's press release is brief, and includes Jobs' email to the company:
Apple CEO Steve Jobs today sent the following email to all Apple employees:
At my request, the board of directors has granted me a medical leave of absence so I can focus on my health. I will continue as CEO and be involved in major strategic decisions for the company.
I have asked Tim Cook to be responsible for all of Apple’s day to day operations. I have great confidence that Tim and the rest of the executive management team will do a terrific job executing the exciting plans we have in place for 2011.
I love Apple so much and hope to be back as soon as I can. In the meantime, my family and I would deeply appreciate respect for our privacy.