Personally, I don't think Apple is really worth that much. Their Price/Earnings ratio is 20.8 to 1, compared to 13.5 to 1 for Microsoft and 13.6 to 1 for Exxon. That means Apple stock costs about half again as much for each dollar of income as Microsoft or Exxon. Apple's net income was $8.2 billion in 2009, compared to 14.6 billion for Microsoft and 19.3 billion for Exxon.
While all the stock analysts are claiming Apple stock is only going to go higher, I don't see this happening. The numbers just aren't there. Going by income and P/E, Apple should be worth about $125-150 billion, about on par with Google.
But never underestimate Steve Jobs and the hype surrounding him. When he dies, Apple's stock is going to absolutely tank.