Jobs has been on medical leave since January and Tim Cook has been acting as CEO since then. Jobs just stepped aside completely in August by resigning.
"At my request, the board of directors has granted me a medical leave of absence so I can focus on my health. I will continue as CEO and be involved in major strategic decisions for the company."
edit: I don't know of a better way to describe Cooks role during this period as he was responsible for daily operations while Jobs retained the title of CEO. Acting CEO may not be the correct description.