But it's very success - with the market-leading iPad and the voice-enabled iPhone 4S - is luring cheaper rivals to the surface.

Google Inc's Android, launched a few years ago and taking aim squarely at the high-end iOS, continues to attract cellphone makers. Amazon.com Inc's Kindle Fire, half the cost of the iPad, is expected to have chipped away at the lower end of the tablet market.

Finally, though many on Wall Street, betting that an iTV and 4G iPhones and iPads will again pack its stores, continue to bank on a share-price climb to as high as $700, some begin to question the sustainability of Apple's torrid growth pace.

Apple tacked on $43 billion to its top line in fiscal 2011, lifting it to $108.25 billion - a 65 percent increase from the previous year.

Barry Jaruzelski, a consumer hardware business expert and partner at consulting firm Booz & Co, said to sustain that is effectively to conjure a Fortune 500 company out of thin air - year after year.

"You become a victim of your own success," he said. "Can you grow the existing products that much, or can you create a new category that creates $10 billion to $20 billion? That is the challenge."

When Apple reports earnings January 24, many investors for the first time might be watching for chinks in the armor, especially given Apple's first revenue miss since 2008 for the October quarter.

"The risk is the sustainability of what they have been doing," said ISI Group analyst Brian Marshall. "They have put up a huge number and the question is can they continue to penetrate with their current existing product portfolio at these price levels?"

The fear is that the number of people who can afford an iPad or an iPhone is dwindling, he added.

RIVALS HOT ON HEELS

Apple has gone on a tear the last few years.

With $81.6 billion of cash, surging sales across product lines - most notably its best-selling iPhone and iPad - and fevered anticipation that it might make a big, game-changing bet on TV, many still say Apple has only one way to go this year: up.

As one of the companies that is a leader in major trends in technology - mobile connectivity and the cloud - Apple's revenue is expected easily to rise 30 percent this year and nearly 50 percent in its fiscal first quarter.

The average estimates for sales of Apple's products during the fiscal first quarter, which includes the holiday shopping season, are roughly 31 million iPhones, 13.5 million to 14 million iPads and 5 million Mac computers. But investors wouldn't be surprised if Apple handily beats these estimates.

Apple's stock trades at about 15 times earnings, versus 10 times for Microsoft and 21 times for Google. Some argue for excluding Apple's massive, $80 billion-plus in cash and investments from the valuation, meaning Apple trades at a much lower multiple.

"This is just a stepping point for it to go another 15 to 20 percent higher than it is now," said Michael Yoshikami, CEO of YCMNET Advisors, which owns Apple shares, adding that international expansion will drive much of the upside.

"The stock is cheap relative to companies like Google. It's a good value, especially considering what a growth trajectory this company is on."

Its stock gained 25 percent in 2011, adding about $77 billion to Apple's market cap, and touched an all-time high of $431.36 this past week. That's a remarkable run for any company in a volatile stock market, yet the stock is way off from an average expectation for about $550.

Increasingly formidable competition and the pressure it could bring to bear on margins may be part of the story. The $200 Kindle, for example, is sold at a loss by Amazon as it tries to get a toe-hold in the tablet market.

But Apple could employ its successful iPhone pricing strategy, along with its mighty supply chain, to the iPad to counter cheaper rivals and maintain its dominance, said Michael Walkley, an analyst with Canaccord Genuity.

Apple could "offer iPad 2 at a reduced price of roughly $350", he said, adding that the new iPad 3 could drive a high-tier tablet upgrade cycle with the discounted iPad 2 driving sales into lower price points.

According to the latest research from Nielsen, Apple is also closing the gap on Android, with 44.5 percent of people who acquired a smartphone recently saying they bought an iPhone when asked in December, compared with just 25.1 percent in October.

But Android continues to lead with 46.3 percent of all smartphone owners surveyed last quarter reporting they have an Android-based mobile phone, while Apple had a 30 percent share.

Investors, though, by and large still see the iPad and iPhone as the far superior products in their classes.

"We also expect continued iPad strength, though refresh timing could create a sales gap," said William Power, an analyst with Baird Equity Research.

"Although we do expect more meaningful tablet competition from Amazon and others this year, including the Kindle Fire, we believe the iPad remains best positioned at the higher end."

"I'VE CRACKED IT"

For now, Apple's bulls hold sway, with 50 of 55 analysts covering the stock rating it a Strong Buy or Buy. Among its advantages are the global spread of the iPhone, which should sell more than 130 million units this year, and the mystique of an iPad that a plethora of rivals from Hewlett Packard to Research in Motion have not been able to best.

Apple's MacBook Air has spawned a whole industry of thin and light laptops that everyone from Intel Inc to HP to Asian computer makers is trying to match.

Some observers are now willing to bet that Apple can indeed pull a rabbit out of its hat with an "iTV", thereby producing a new multibillion-dollar growth business.

Many expect Apple to launch a voice-controlled TV in the second half, one of the most talked about topics at this year's Consumer Electronics Show in Las Vegas.

"At CES many (manufacturers) showcased smart TVs, which we think were quite good first shots (often with gesture and voice control integrated), but are unlikely to match the seamless user interface of the upcoming iTV," said Peter Misek, an analyst with Jefferies.

Apple has neither confirmed nor denied that it was working on a TV, but late co-founder Steve Jobs did reveal to biographer Walter Isaacson his interest in reinventing the television set. "I finally cracked it," he told Isaacson.

Apple is expected to report earnings of $10.07 a share on revenue of $38.76 billion, according to Thomson Reuters I/B/E/S. Wall Street is also forecasting a gross margin of 40.76 percent, up from the previous period.

(Editing by Edwin Chan, Gary Hill)

(This update corrects the eighth paragraph to show Apple missed revenue expectations in 2008, not 2004)

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