If you just look at how thin he is, you'd know it. But now Steve Jobs himself has admitted that his declining health is keeping him from taking the Macworld stage tomorrow.

In a letter posted to Apple's website, Jobs writes:

As many of you know, I have been losing weight throughout 2008. The reason has been a mystery to me and my doctors. A few weeks ago, I decided that getting to the root cause of this and reversing it needed to become my #1 priority.

Fortunately, after further testing, my doctors think they have found the cause-a hormone imbalance that has been "robbing" me of the proteins my body needs to be healthy. Sophisticated blood tests have confirmed this diagnosis.

The remedy for this nutritional problem is relatively simple and straightforward, and I've already begun treatment. But, just like I didn't lose this much weight and body mass in a week or a month, my doctors expect it will take me until late this Spring to regain it. I will continue as Apple's CEO during my recovery.

The letter is an obvious response to the furor unleashed by a post on the gadget blog Gizmodo, in which a source privy to Apple's product secrets claimed Jobs's health was "rapidly declining." The source was right about the "declining" part, wrong about the "rapidly," it turns out.

It has been an expensive secret to keep. Apple's market value has jumped by $2.3 billion this morning alone, as traders reveled in the relief of a lie undone.

In the fall of 2003, Jobs was diagnosed with pancreatic cancer, a fact he kept secret until July 2004, when he underwent surgery to remove the cancer and rewire his digestive system.

This summer, when Jobs's gauntness grew unmistakable, people wondered if Jobs's cancer had returned. But a simple look at a decade worth of photos of Jobs, who is famed for his annual keynote speech at January's Macworld Expo, showed that his weight loss was progressive, not the sign of some sudden illness.

Stupidly, that was the story Apple spokespeople stuck to — that he had a "common bug." The denial of a deeper health problem just fueled more speculation — and led Jobs, in a profane call to a New York Times writer, to explain, off the record, the same mysterious health problem he's now told the world about.

Jobs's terse explanation, though, raises more question than it answers. Now that he's given everyone a tidbit, they'll want more. Which hormones? What's the cure? Why did Jobs take so long to say he had a health problem? Why did Apple PR lie and say everything was fine? And why did gullibly servile reporters like CNBC's Jim Goldman buy it, counting a call to an Apple spokesman as "reporting"?

Which brings me back to the stock market's reaction: Investors are, perversely, happier knowing Jobs is unwell. Which means it's time for him to go. He has done incredible things for Apple in the past decade, rescuing a company that the industry viewed as certainly moribund.

But the lack of clarity about his future has become an albatross for the company. The stock market hates risk. So the possibility that Apple might lose Jobs's services through death and illness gets discounted; as far as investors are concerned, Jobs is already dead to them.

Which is the real reason why we're seeing Apple marketing executive Phil Schiller, Jobs's jokey demo sidekick, give Jobs's keynote speech tomorrow. Jobs may be better by the spring. But it's time for his company to act like he's gone.