Apple reported yesterday their 3rd Quarter earnings and revenue was up 38% compared to a year ago. Additionally Mac sales reached an all-time high. The Apple stock fell almost 10% in after hour trading because investors are not pleased with the earnings forecast for the 4th quarter.

Apple argues that cross margins will fall as Apple introduces new unnamed products at price points rivals will not able to match.
An additional reason for why investors put the Apple stock under pressure is apparently the unclear situation about Steve Jobs health. Apple did not comment on the health situation. Steve Jobs is seen very much as a big part of Apple and its success. If he would have to leave the company there is currently no "plan B" visible to the outside that would give investors confidents that the Apple success story can continue.
See the Apple earnings release for full details on the sales numbers.