Yesterday, while the markets were closed for Martin Luther King Jr. Day, Steve Jobs announced that he is taking another medical leave, raising new concerns among investors about the company’s future. During his leave, Apple chief operating officer Tim Cook will be responsible for all of the company’s day to day operations.

“At my request, the board of directors has granted me a medical leave of absence so I can focus on my health,” Jobs said in an email to all Apple employees, which was published on the company’s website. The email did not disclose details about his health condition or when he is expected to resume full command.

This leave marks the third time in the past ten years that Jobs has stepped back from his role for medical reasons. He took medical leave in 2004 and then again in the first half of 2009, returning to the company in late June of that year.

Today when the stock market opened for trading, AAPL fell as far as $326.00 before recovering to the current PPS of $336.85, still down 3.34% from Friday’s close. Analysts say that Jobs’ leave shouldn’t derail the company’s current momentum, but could have longer-term implications.