It’s almost as if Apple had a long-range secret plan to lull Microsoft into a false sense of security while it quietly rebuilt its ability to compete for OS turf.

This week research firm NPD Group announced that Apple dominated the U.S. retail market for high-end computers in the first quarter of 2008, selling two out of every three PCs priced over $1,000.

Echoing trends seen in other market data, NPD said Mac laptop sales saw year-over-year growth over 50 percent, while desktop sales grew 45 percent. During the same period the Windows PC market struggled with zero growth in laptop sales and a 25 percent decline in desktop sales.

NPD estimates the Mac’s overall U.S. market share at 13.8 percent, up from 9.5 percent a year ago.

Before I continue, some caveats: the NPD data dos not include online sales, which is how Dell sells most of its PCs. Nor does it include enterprise sales, where Apple has a negligible presence. And it must be noted that Apple has but one Mac offering in the sub-$1,000 category: the Mac Mini, a small computer that sells without a monitor, keyboard or mouse.

Nevertheless, the NPD data makes it clear that U.S. consumers willing to spend a little more for a computer are choosing the Mac over Windows PCs in rapidly climbing numbers.

Now add this news to last week’s announcement from Microsoft that Office 2008 for the Mac is “selling faster than any previous version of Office for Mac in 19 years.” Since its January launch, the product has sold at three times the rate of the previous Mac release, Office 2004.

The tremendous uptick in sales for the latest Mac version of Office closely mirrors the increase in Mac unit sales since 2004 – Apple sells three times as many Macs now as it did then, with the numbers mounting steadily every quarter.

Until recently, the rise in the Mac’s market share was portrayed merely as stabilization of the Mac OS platform. But the accelerating rate of the Mac’s share and especially the more affluent consumers it continues to lure from Windows is bound to take a toll on Microsoft eventually.

As more consumers buy Macs over PCs, Microsoft sells fewer copies of Windows. True enough, some Mac owners (particularly switchers) do buy retail copies of Windows to run alongside Mac OS X, but not all – probably not even a majority.

Also true: Microsoft makes a boatload of money from Office for the Mac. Still, a significant shift in the Mac/Windows dynamic would have an impact beyond the dollars spent on software.

Think about it. The Mac’s gains so far already have destroyed the psychology that developed in the 1990s that Windows is the only reasonable choice for the average PC buyer.

Thanks to the success of the iPod in establishing Apple as the brand of cool tech, as well as the company’s ever-growing retail presence in upscale shopping malls, PC shoppers are far more likely to consider buying a Mac today than they were three years ago.

Once Apple got the Mac back on the public’s radar screen, its ability to deliver on consumer expectations fed the momentum. The Mac has established itself as not just an alternative to Windows, but now for many a preferable alternative.

What intrigues me are the consequences of the Mac as a legitimate competitor to Windows in the marketplace for the first time in over 15 years (one could argue the first time ever).

I don’t envision a day when Apple vanquishes Microsoft from the OS battlefield, but that won’t be necessary.

If Apple succeeds in winning 20 to 25 percent of the market, Microsoft would lose one of its key assets – the leverage that comes from holding a near-monopoly on PC operating systems. For years Microsoft used that power to bully rivals and promote its own technologies over that of would-be competitors (think Internet Explorer).

Even if Apple’s market share levels off in the low 20’s, it would seriously hamper Microsoft’s ability to impose its will on the computer users both in the home and the workplace.

And what about the workplace? As the Mac becomes the computer of choice in more homes and on more college campuses, more young people entering the work force will question the primacy of Windows in the business world.

Although Apple has so far declined to target business customers, the popularity of the iPhone among executives along with changing expectations of the iPod generation could result in the Mac taking inadvertent bites out of the enterprise market.

Such a turn of events would start eating into Microsoft’s profits. With Apple nibbling at the Windows hegemony and Google jeopardizing the dominance of Office with Google Apps, Microsoft could face threats to both of its cash cows. (Office and Windows comprise about 70 percent of Microsoft revenues).

Not that Microsoft will disappear any time soon, but it would be nice to see them get the wake-up call that mediocrity will not carry the day forever. True competition in the areas Microsoft has dominated for over 20 years would reward consumers with better products from all parties.

Innovation – something about which Apple knows a thing or two -- might matter again.