From the New York Times:
Here is the rub: Apple is so big, it’s running up against the law of large numbers.
Also known as the golden theorem, with a proof attributed to the 17th-century Swiss mathematician Jacob Bernoulli, the law states that a variable will revert to a mean over a large sample of results. In the case of the largest companies, it suggests that high earnings growth and a rapid rise in share price will slow as those companies grow ever larger.
If Apple’s share price grew even 20 percent a year for the next decade, which is far below its current blistering pace, its $500 billion market capitalization would be more than $3 trillion by 2022. That is bigger than the 2011 gross domestic product of France or Brazil.
Considering the large market opportunities left for Apple in Phones (8% of the world marketshare) , PCs (5%), Tablets (70% plus but growing) and possible new products in the future there is a lot of steam left. For more read Asymco.