"By now, everyone probably knows that Apple (AAPL) CEO Steve Jobs will be taking another medical leave of absence," says Geoffrey Seiler.

The editor of BullMarket.com explains, "We'd use the weakness in Apple shares to accumulate the stock. The last time Job took medical leave was in 2009 when the stock was trading in the mid $80s.

"Chief Operating Officer Tim Cook will take over the day-to-day operations while Jobs is on leave. Cook previously filled in for Jobs in 2004 when Jobs was battling pancreatic cancer and in 2009 when Jobs was recovering from a liver transplant.

So it looks like COO Tim Cook will be in charge again over at Apple (AAPL). CEO Steve Jobs said Monday that he is taking another medical leave of absence, leaving responsibility for day-to-day decisions to Cook.

This is the third time Cook took up the reins, the first in 2004 when Jobs recovered from surgery for pancreatic cancer, and again in January through June of 2009 when Jobs had a liver transplant. Many people consider Cook to be Jobs's logical successor.

Apple Inc. (NASDAQ: AAPL) announced today that its CEO and co-founder, Steve Jobs, is back at work following a five-and-a-half month medical leave for a liver transplant.

According to Apple, Jobs is going to be working a few days a week at the company's headquarters, and working from home the remainder of days.

Now that Apple, Inc.'s (NASDAQ: AAPL) biggest day of 2009 is officially over, co-founder and CEO-on-leave Steve Jobs did not make an appearance. Although he's scheduled to come back to his day-to-day duties at Apple at the end of this month, some thought he may pay a visit to Apple's worldwide developer's conference to kick off a new iPhone or any other iDevice. It didn't happen, and Apple hasn't even told the world if plans for Jobs to return in a little over two weeks are still on track.

Apple Inc. (NASDAQ: AAPL) new Chief Executive Tim Cook seems the type who enjoys lurking in the background. For years, he has quietly but effectively undertaken some of Apple's biggest jobs including running the Macintosh business while allowing co-founder Steve Jobs to bask in the spotlight. With Jobs going on leave, Cook will have to step in front of the curtain again.

He ran the show in 2004 when Jobs was treated for cancer. Bloomberg News describes Cook as a soft-spoken yet intense manager who, like his boss, does not suffer fools gladly.

But he is not going to be able to inspire the cult-like devotion of Jobs. Investors are understandably worried.

Jobs' health crisis could not come at a worse time for the Cupertino, Calif.-based Apple. The economy is slowing as are sales of computers. As the Wall Street Journal noted "IDC recently reported that world-wide computer shipments fell in the fourth quarter from a year earlier, the first year-to-year drop in six years." Sales of iPods are expected to decline, an indication of the maturity of the market.

Apple still has plenty going for it. The company's U.S. computer shipments rose 7.5% and its share of computer shipments rose to 7.2% from 6.4% a year earlier. It continues to have a rabidly loyal customer base including me.

But there is going to be a cloud over Apple for as long as Jobs' health is an issue.

Well it turns out that Steve Jobs is going to have to spend a bit more time than he thought dealing with his "hormonal imbalance." It pains me to write this, but Jobs is taking a leave of absence from Apple Inc. (NASDAQ: AAPL) CEO's slot until June -- at the earliest -- and his chief operating office, Tim Cook, will take over as CEO in the interim.

I don't know how it turned out that Jobs needed to spend more time than he previously thought dealing with his hormonal imbalance. But I really hope he recovers and can return to the company in June. Unfortunately, investors do not have enough information to know whether there is any chance of this really happening.

In the meantime, they will have half a year to see whether Tim Cook can carry the Apple on his head. In after-hours trading, Apple stock is plunging -- down at least 8%.

Peter Cohan is President of Peter S. Cohan & Associates. He also teaches management at Babson College and edits The Cohan Letter. He has no financial interest in the securities mentioned.

Apple (NASDAQ: AAPL) has been on the rise today, as renewed enthusiasm over future iPhone sales has brought buyers into the stock, pushing shares up $7.67 to $130.63, or 6.2% .

The company has stated that its goal for overall iPhone sales by the end of 2008 was 10 million units, and according to Apple's COO, Tim Cook, the company remains confident in hitting that hefty goal.

Since the highly anticipated release of the iPhone last year, there have been a couple of points that Apple has taken a bit of heat over, the first being that outside programmers were not allowed to write programs for the iPhone, and the second being the company's decision to grant individual carriers rights to sell their phones in their respective countries.