While Apple is certainly worth a good deal of money, a $100 move in Apple stock represents more money changing hands than the entire market caps of GE (GE), Salesforce (CRM), and Netflix (NFLX) combined.
The valuation on Apple looks pretty reasonable but we are moving into the weak part of the year for their sales, and many consumers will likely skip a generation of phones and tablets if the recent move in gadgets was cyclical and not a permanent migration towards consumers buying a new device quarterly. In any event, my point is that Apple has moved very far, very fast and the stock could correct 10-20% without any major changes in growth rates or valuation would take place. The investment community, on the other hand, would be completely hopeless and desperate if Apple were to ever close below $600 let alone $500 again.
Because no one thinks it could happen, it just may happen and happen fast. Take a look at how overbought this stock is! Should Apple pullback to its 200 day moving average (and that happens to most stocks sooner or later), the stock would have to hit $400 a share.