Wall Street analysts expect Apple will announce robust iPhone sales when the iconic device maker reports its financial results for the final three months of 2011 on Tuesday.

According to the estimates submitted by several investment firm analysts, Apple's iPhone sales during 2011's seasonally strong fourth quarter are on track to land somewhere between 25.2 million units and 32 million units. Piper Jaffray analysts Gene Munster and Andrew Murphy, for example, are now modeling for Apple to have sold 30.6 million iPhones.

"We are increasingly confident in Dec. quarter iPhone 4 and 4S sales," Munster and Murphy wrote in a recent investor note. "Our previous estimate of 30 million could prove to be conservative."

Piper Jaffray's analysts also noted that their analysis of the latest data from NPD concerning domestic gadget shipments suggests that Apple's fourth-quarter iPod sales had also exceeded their expectations. "Our model at 15.5 million iPods may prove to be conservative," Munster and Murphy wrote.

Healthy Mac Growth

Based on the latest domestic PC data from the NPD Group, Piper Jaffray estimates that Apple's global Mac sales during the fourth quarter reached 4.9 million units -- an 18 percent rise from the comparable quarter in 2010.

Still, the revised estimate from Munster and Murphy is down from the investment firm's prior projection of 25 percent growth, due to softer than expected sales in the U.S. market during last year's final quarter.

"However, we are increasingly confident in [Apple's Dec. iPhone sales] and are raising our unit estimate by 600,000 units, fully offsetting the reduced Mac revenue," Munster and Murphy wrote. "We estimate Mac revenue will represent 16 percent of overall revenue [for Apple] in the Dec. quarter," they added.

High consumer interest in tablets -- especially Apple's iPad 2 and Amazon's Kindle Fire -- may be one reason why Apple's Mac shipments appear to have been lower than analysts had initially expected for the fourth quarter. According to a new study by the Pew Research organization, the share of adults in the United States who own e-readers or tablets nearly doubled from 10 percent in mid-December to 19 percent in early January.

"The media tablet impact [on PC sales in the final three months of 2011] was very hard to quantify," said Mika Kitagawa, a principal analyst at Gartner. "We think that media tablets were not the only items" that affected PC sales, and with "other items like smartphones" likewise having had an impact on consumers' holiday shopping decisions.

Apple's Ultrabook Challenge

According to Kitagawa, new Ultrabooks based on the latest ultra-low-power processors and chipsets from Intel did not appear to have had a major impact on the mobile PC segment overall, or on Apple's Mac sales in particular. "Ultrabooks didn't seem to draw consumers' attention" during this year's holiday shopping season, Kitagawa said.

One reason why is that Intel and PC vendors did not heavily promote their available Ultrabook offerings in the final three months of last year. "Consumers had very little understanding and awareness of Ultrabooks, and only a small group of consumers was willing to pay the price premium for such models," Kitagawa said.

Looking forward to the year ahead, however, Apple's notebook lineup is expected to face increased competition from the avalanche of new Mac Air lookalike Ultrabooks that rival vendors unveiled in Las Vegas earlier this month. Rival notebook vendors "want to market Ultrabooks heavily in 2012 and [used] CES as a big Ultrabook debut stage," Kitagawa said.

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