Apple's Chain Reaction

"This allows them to reach out to consumers who may not normally or necessarily be interested in Apple," says Tim Deal, an analyst with industry consulting firm Technology Business Research.

On the other hand, the stores have not been an unqualified success. The stores division showed an operating loss until last year, and even then, much of the operating income would have been erased by depreciation and amortization charges.

Costs also have jumped from leasing new properties and building storefronts from raw space, although the growth in retail revenue has outpaced those costs so far.

But the longer-term costs may be more of an issue. The company has been able to show such rapid growth from retail partly because of the relative youth of its stores; more than half the stores are less than two years old.

Typically, in the retail industry, individual stores show their greatest sales growth in the first four years before that growth starts to tail off, notes Johnson. On a broader level, as Apple's stores age, those older stores will become an increasingly larger portion of the company's overall store base, further weighing on the company's retail growth prospects.

Apple's ability to maintain its retail growth also may be hit by other trends. The company so far has focused on putting stores in affluent communities and in high-end malls.

But it soon will run out of such desirable locations, according to analysts. The company may find that it either has to slow down expansion or put stores in less-favorable areas.

"I don't expect them to blanket the entire U.S. or Europe," Del Prete says. "In the absence of being able to open a lot of stores, the growth will slow down.