According to The Wall Street Journal, some of Apple's board members have been informally discussing an eventual successor to Steve Jobs since the CEO began his latest medical leave earlier this year. These board members "have discussed CEO succession with executive recruiters and at least one head of a high-profile technology company," though the Journal says they're not actively engaged in recruiting a new CEO.
The Journal notes that not all board members have been involved in the discussion (obviously Jobs himself wouldn't have been), and it also says management succession plans are discussed at every board meeting, though such plans are "sworn to secrecy."
Steve Jobs reportedly responded to the report via email, saying "I think it's hogwash." Either this means the Journal's report is inaccurate in some way, or Jobs may have been kept out of the loop on the discussions. For his part, Jobs has given no indication that he will step down anytime soon, though many financial analysts have grown increasingly skittish following the CEO's ongoing (and over-publicized) medical issues and Apple's intense privacy surrounding the succession plan.