If Apple are not honest about his health then they could surely be guilty of fraud by concealing information likely to affect the stock price ?
Not if federal health privacy laws require that concealment, no matter what the size of the corporation nor the potential effect on the stock market.
Tell you the truth Apple already got looked into once over the matter in the past before Steve's liver transplant where he took a leave then and it was found out after the fact what really happen.
SEC said that Steve Jobs might be a special case at the time as he was the CEO and his health would have a much more drastic effect on the stock price than any other CEO of a publicly traded company. They looked into it seeing if there was any fowl play.
Now that Steven Jobs has complete stepped down as CEO I do not think they have to report it as much as he is not the CEO of the company and as such the public should not have any right to his information. I would not be surprised in the least if the SEC decides to investigated Apple again later on. Big time if the stock drops by a large amount if/when he dies.