Steve Jobs is only the CEO of Apple, he is NOT the god of product design, nor the only reason that Apple has grown so much since his return. Jobs returned from NeXT and Pixar at a point where Apple was quiet, but was lucky enough to have some very talented people hidden in the R&D departments. In contrast to Apple of the 80s, instead of spewing money all over the place in the hope of getting some ok products, Jobs made some canny decisions to cut effort in areas that were never going to make serious money - WebObjects I'm looking at you - it's just a framework for websites, not a money-maker. On the other hand, he and his collegues in management seem to have been very smart in recognising the strength of the iPod and it's simplicity. Of course, the alternative interpretation of the success of the iPod is a mixture of Apple-fanboyism plus just happening to hit the sweet spot in growth in the market for MP3 players.
Now that Apple is successful, and still growing, the fact that Steve Jobs is taking a break (that might turn out to be permanent) really shouldn't be causing everyone to suddenly pull their investment out of the shares. Afterall, Steve Jobs did not design the iPod (nor any of it's variants), nor the current crop of Apple laptops, nor the iPhone.... and someone else will be able to do the same for future Apple products, providing Apple continue to employ excellent designers and product creators.
Now let's let the guy take the time he needs to to get well. He's only human at the end of it all.