If you've got untreatable cancer then yeah you should be forced to disclose. However cancer invokes more fear than there is actual risk, if I get skin cancer its 100% treatable, some forms don't even need treating because its merely a skin growth with no health risk except maybe an infection if you cut it off with a blunt knife. Yet if a CEO announced they had 'skin cancer' the stock would drop, even if it is one of the benign forms that doctors treat with wart removers.
I think saving the investors from themselves should be just as big a concern. He is predicted with survival past 10 years, which IIRC the average age of death with someone having that prognosis is in fact only a few years shorter than the average age of death. If Jobs getting cancer means he kicks it at 78 instead of 82, I don't see why it should be disclosed immediately or at all.
Here is the text of the Commencement address by Steve Jobs in 2005 at Stanford where he announces that he had cancer.
Edit: not agreeing or disagreeing with anyone's opinions on CNN mentioning his pancreatic cancer, I'm just linking this because I was reminded of it after reading the comment about his cancer and I recall it being an interesting read.
I agree. It IS personal. People can use whatever justification they want to pry into seamy details, but unless he was given a date of expiration, it's his information to do with what he wants.