let's break it all down:
first, the stock market. there are three kinds of buyers - traders, speculators, and investors.
traders are the pros who make money whether prices go up or down, using all the short term tricks of the trade. they have a big impact on day-to-day stock price movements (like the exact $500 price close recently), but not long term. right now they are either getting out of Apple or making complex trading bets on where it will be in the months ahead. this is pushing the price down presently.
speculators are the ones seeking "growth" stocks based on hype, hope, or some crystal ball that they will go up in price significantly faster than the overall market within months or a year or two. this is the group that has now abandoned Apple and is dumping it, after propelling it upward recently, which is definitely pushing the price down right now.
investors are the multi-year long-term buyers who seek a modest but steady return at low risk that is still better than alternatives like bonds and other financial instruments. pension funds, mutual funds, etc. they pay attention to fundamentals like P/E ratios. they are holding Apple now and will start to buy soon when it stops falling.
second, Apple's business (here are some great charts: http://www.splatf.com/2013/01/apple-dec12earnings-charts/):
as many above note, Apple has gone through a spectacular revenue/profit growth phase in the last five years thanks to the addition of two new hugely successful products, the iPhone and the iPad. it previous two main products, the Mac and iPod are now just modest contributors to its revenues.
so to remain a "growth" stock attractive to the speculators, Apple would need to come out with yet another new groundbreaking product to add to what it has now - every three years! i doubt that is possible in any event. but maybe every five years ... we'll see. we all have our notions about what that could be.
but in any event there are solid prospects for steady and substantial growth in sales for all Apple products except iPods for years ahead. the near-universal shift to portable computing devices is only partly done, and global markets for smartphones/iPhone and tablets/iPad are clearly not yet "saturated" (China!). these are not "mature" markets - that will take 5-10 years - and Apple will continue to take a big chunk of their significantly expanding new revenues until it is.
so successful competition is the only real identifiable risk for Apple in coming years. and that is really up to Apple's leadership team to, indeed, keep making the best products they can. for me, the key indicator of that this year will be iOS 7 and iCloud. both need significant improvements to keep up and surpass the competition. we'll see ...