Apple employees received a somber note from CEO Steve Jobs this morning:

“Team, 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.”

The email message continued to explain that chief operating officer Tim Cook will be taking over Apple’s day-to-day operations – marking the third time in seven years. “I love Apple so much and hope to be back as soon as I can,” he added. “In the meantime, my family and I would deeply appreciate respect for our privacy.”

Jobs is a cancer survivor and stepped away from the company for nearly six months back in 2009 to undergo a liver transplant. Although Jobs has always treated his health as a private matter, many industry observers have argued that one of the world’s most famous CEOs has a duty to the public (and Apple shareholders) to come clean about his health.

In fact, New York Times writer Joe Nocera sparked a heated debate back in January 2009 when he wrote in his Executive Suite column: “There are certain people who simply don’t have the same privacy rights as others, whether they like it or not. Presidents. Celebrities. Sports figures. And, at least in terms of his health, Steve Jobs. His health has become a material fact for Apple shareholders. His vagueness about his health, his dissembling, his constantly changing story line — it is simply not an appropriate way to act when you are the most important person at one of the most high-profile companies in America. On the contrary: it is infuriating.”

In the past, news of Jobs’ health have caused shares of Apple to plunge.