Good commentary [pdf]:
"When we look at our cash flow models, assuming Apple can maintain its current operating margins (a heroic assumption in the face of increased competition in the tablet market), to justify the current stock price, it appears to us that Apple will have to sell about $2.6 trillion worth of total products and services over the next ten years. Last year’s revenues (for the fiscal year ending 9/24/11) totaled $108 billion. [...] Since not all 310 million people in America use Apple, those who
do need to spend a lot more and the vast majority of those sales will need to be on devices because iTunes sales do not bring much profitability."