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Apple (Nasdaq: AAPL) investors eagerly waiting to see the iPad 3's specs on March 7 may be better served listening to how the company plans to ramp up its sales efforts.

Sure, the sleek tablet has been in hot demand, but its unit-shipment performance has lagged behind its older brother, the iPhone. And given that iPad sales contribute roughly 20% of total revenues to Apple, keeping the growth machine humming is a good thing.

Crunching some tasty numbers
iPad shipments are expected to reach an estimated 53 million units by the end of Apple's fiscal year -- or at least that's what Wall Street soothsayers such as Ashok Kumar of Maxim Group are saying. That seems a likely figure, given that Apple shipped 15.4 million iPads during its quarter that included December -- a period when a number of retailers and manufacturers of consumer goods post their strongest sales of the year.

For Apple, shipping 53 million iPads in fiscal 2012 would translate into a 64% increase over its first full year of iPad shipments in fiscal 2011. Not bad, you might say. But consider this: Apple scored a 78% iPhone increase in fiscal 2009, when it shipped 20.7 million of the snazzy smartphone. And in its most recent blowout quarter, Apple posted a staggering 128% increase in iPhone shipments over year-ago figures, when it moved a whopping 37 million units in the first quarter. The iPad, in contrast, generated a 111% increase in shipments during the same period.

What makes those first-quarter figures particularly interesting is that the iPhone is a more mature product line, and typically it's harder for an older item to achieve higher growth rates over a newer product line, like the iPad, which is working from a smaller base of units sold.

Yet investors are seeing stronger unit-shipment growth with the iPhone when comparing the two mobile devices along the same lifecycle. Assuming iPad shipments grow by an additional 20 million next year, similar to the level expected for this fiscal year, that would yield a less-than-sexy annual growth rate of 40% in 2013.

And between now and next year, there are a few issues that could drive that growth rate even lower, regardless of an iPad 3 arrival. That's because competitors are getting smarter at, well, competing.

iPad vs. the others
Although the iPad 2 claimed the lauded title of best tablet at the Mobile World Congress in Barcelona, Spain, Samsung's Galaxy S II beat out the iPhone 4S in the smartphone category, according to a PC Magazine report, which also said the South Korean manufacturer snagged the MWC title of device manufacturer of the year. It's not unfathomable to believe that Samsung will eventually outdo the iPad.

Meanwhile, in the low end of the tablet market, where Apple lacks a presence, (Nasdaq: AMZN) has the Kindle Fire and Barnes & Noble (NYSE: BKS) offers the Nook Tablet -- each of which have models that retail for $199. That's substantially lower than the iPad's $499 starting price.

While some folks believe it wouldn't be surprising to see Apple bite the bullet and lower its price on the iPad 2 once the new iPad is unveiled on March 7, iPad fence-sitters are likely to turn their backs on Apple if it doesn't come close to rivaling its bookish competitors Amazon and B&N on price.

Add to that mix a potential dark horse this year, as ultrabooks make their debut armed with Windows 8. If these units hit the consumer sweet spot of $500 or less, they may give iPads -- not to mention other tablets -- a run for their money. After all, snapping up a feather-light ultrabook that has a similar weight to an iPad yet comes with more functionality could be a proposition that consumers find hard to beat. According to a PCWorld report, Intel is aiming to deliver a Windows 8 ultrabook later this year that could be priced as low as $699.

Foolish takeaway
While the iPad's growth rate may continue to lag behind the iPhone, investors can take some solace in knowing that it probably has a long way to go before it begins to suffer negative growth numbers, as the pioneering iPod now does. In the meantime, The Motley Fool has compiled a free special report outlining the "3 Hidden Winners of the iPhone, iPad, and Android Revolution." It highlights three hidden component plays that are profiting from the smartphone revolution yet are are off most investors' radars. Download the report now.

Fool contributor Dawn Kawamoto owns no shares in the companies listed. The Motley Fool owns shares of, Intel, and Apple. Motley Fool newsletter services have recommended buying shares of, Intel, and Apple and creating a bull call spread position in Apple. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.