Apple shares down after Jobs' reversal on health

Apple spokesman Steve Dowling would not elaborate on Jobs' condition or what he discovered in the past week.

"They'll tell you the least they can tell you," longtime industry analyst Roger Kay of Endpoint Technology Associates said. "They're trying to have it both ways, to protect their guy's privacy and feelings and at the same time somehow signal the market."

Cook, who ran Apple for two months in 2004 when Jobs was recovering from his cancer surgery, is seen as one of Jobs' most likely successors, along with Apple's top marketing executive, Philip Schiller. American Technology Research analyst Brian Marshall — who last week predicted Jobs would step down this year — said Wednesday's announcement tips the bets in Cook's favor.

"The company has been soft-signaling to the Street for a while now that Steve Jobs is not going to be CEO forever," he said. "This will be sort of a trial period for Cook to be chief executive."

Cook, 48, lacks Jobs' charisma and showmanship, but is seen as a solid pick.

"Tim Cook is a very experienced and highly regarded chief operating officer," said Calyon Securities analyst Shebly Seyrafi. "He's qualified."