Steve Jobs takes another medical leave

Reasons are vague. No end-date given. COO Tim Cook will once again step in to run Apple.

The text of his letter to Apple’s (AAPL) staff, issued Monday morning as a press advisory:

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.

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.

Steve

Fasten your seatbelts, it’s going to be a bumpy ride on Wall Street.

The announcement, made on a business holiday the day before the company is expected to announce strong first-quarter earnings, will come as a shock to investors.

Although Jobs’ health problems are legendary — surgery to remove a cancerous tumor from his pancreas in 2004, a liver transplant nearly two years ago — he has seemed to be on a rebound. Since he returned from his last medical leave in the spring of 2009, he has been an increasing visible presence, both in his e-mail exchanges with users and on stage at press events.

But close observers of his public appearances note that his voice is hoarse, his weight less than it should be and that underneath his trademark jeans he wears long underwear, perhaps to maintain his body heat.

With COO Tim Cook running the show, Apple is in good hands. Each time he has stepped in for Jobs, the company’s performance — already pretty strong — has improved.

But that won’t stop speculators from punishing the stock when the market opens Tuesday.