Some stock analysts are scaling back their estimates for iPad sales

Another reason for a possible slowdown in growth is because some buyers are waiting for the iPad 3, expected early in 2012. Some analysts also speculate that Apple has won the early adopter market but will find it harder to entice the next round of buyers.

ANALYSIS: Tablet throwdown: Amazon Kindle Fire vs. Apple iPad 2

So far this month, three analysts have trimmed their iPad forecasts for the current quarter, according to Investor's Business Daily. Keep in mind that all three expect strong iPad sales for this period, just not as much as they expected previously. Shaw Wu, of Sterne Agee, dropped his forecast to 13.5 million units from 15 million, the largest cutback. Michael Walkley, of Canaddord Genuity, lowered his target to 13 million, from 14 million. Mark Moskowitz, of J.P. Morgan, trimmed his estimate the least: to 13 million, from 13.3 million.

"Maybe the tablet market is not as big as people had thought," Wu said, quoted in the IBD story. "Remember all the talk about how tablets are going to take over PCs? I'm not in that camp. A lot of people were. Maybe not anymore."

Another variable is the expected success of Amazon's new Kindle Fire media tablet, despite having received mixed reviews by pundits and tech analysts. This week, Morgan Stanley analyst Scott Devitt said that many reviewers "strategically misunderstood" the new tablet. Amazon is not trying to match the iPad, he says. Instead, it's creating a "large content ecosystem" that plays well with the low-priced Kindle. That combination, he says, "will more than make up for its technical inferiority to the iPad."

Part of the appeal is the price difference. The Kindle Fire, with a 7-inch screen, is $199; the 9.7-inch iPad 2 starts at $499.

But Forrester Research recently predicted that Amazon would sell about 5 million of the new models by Jan. 31, 2012, less than half of projected iPad sales, but enough to make it a very distant No. 2 in the tablet market behind Apple.

Analysts are still bullish on Apple's current quarter. Several, according to the IBD story, have boosted their profit and sales projections due to stronger sales of iPhones and Mac computers.