In only a couple hours of trading since Steve Jobs announced his resignation as CEO, Apple shares have only gone down about 5%. Considering Jobs' impact and influence in the minds of both consumers and Wall Street, that's not really the huge drop that everyone expected.
Steve built up Apple as a well-oiled machine before he stepped down, and he's left that machine in the capable hands of Tim Cook and the rest of the Apple board. It should also be noted that Steve will still be heavily involved in Apple, as he as accepted the postion of Chairman of the Board.