It may be because of the halo effect from the iPhone (and later the iPad), the perception that Macs are less vulnerable to viruses, a rejection of Microsoft (NASDAQ:MSFT), or perhaps the effect of Apple’s increased retail visibility thanks to an expanding network of Apple Stores. Whatever the reason, Apple (NASDAQ:AAPL) has shipped computers at a rate that outperformed Windows PCs for the past five years. The Cupertino, Calif., company reached a symbolic milestone in 2011 when sales of new Macs accounted for over 10% of all PCs sold in the U.S. — a threshold Apple last passed in 1991.

Despite this apparent success, speculation has ramped up in the past year that Apple is planning to kill off OS X, and the Mac with it. Gizmodo fanned the flames with an article headlined “Another Chrystal Clear Sign That OS X Is Going to Die Really Soon,” where it was noted that Apple no longer lists a vice president for OS X or for Mac technologies on its corporate leadership webpage. You can see it here.

The latest iPad offers display resolution, communications capabilities and processing power that’s sufficient to challenge computers for many tasks, and the iPad is outselling the Mac by a factor of three to one, so might the device not supplant the Mac altogether?

Shifts in the Mac turf

With Lion and then Mountain Lion versions of OS X — the operating system used by its Mac computers — Apple has continued to push the platform closer to iOS, the operating system used by its mobile devices. And while there have been new iPads in 2010, 2011, and 2012, a quick scan of Apple’s desktop PC lineup reveals some interesting information:

• In 2011, Apple discontinued its Xserve line of rack-mounted enterprise server PCs, which had been introduced in 2002.
• The Mac Pro, Apple’s only user-upgradable and user-expandable desktop PC (with a starting price that ranges from $2499 to $4999), has not been updated since summer, 2010.
• The iMac, while it has had spec upgrades, hasn’t had a significant appearance refresh since 2007.

Apple’s desktop PC sales and MacBook notebook computer sales have been going in opposite directions for a decade. According to sales figures published in Gigaom, 2003 was the last year that Apple sold more desktop computers than laptops, with desktops holding a 60% share (down from 70% in 2001). In 2004, laptops just edged out desktop PCs by a few percentage points, and by 2010 Apple was selling two laptops for every desktop PC.

In Apple’s Q1 2012 conference call, CEO Time Cook reported that sales for the quarter had included over 37 million iPhones, 15.4 million iPads, and 5.2 million Macs. Desktop PC sales had improved by 21% year over year, with laptops posting a 28% gain. The top sellers in the Mac lineup? According to CFO Peter Oppenheimer, that would be the MacBook Pro, MacBook Air, and iMac.

Operating-system unity

Despite the theories of iPad desktop domination, it’s highly unlikely that the Mac is going anywhere anytime soon.

What does seem likely is a continued convergence between OS X and iOS, perhaps eventually reaching the point where Apple is maintaining only one operating system instead of two. There is significant potential for consolidation in the Mac lineup, though. The MacBook Air and MacBook Pro product lines could be merged (Gigaom and others have been speculating about this possibility lately). Ultrabooks are receiving a huge push on the Windows side and the MacBook Air is a leader in the segment; Apple will not willingly lose market share there.

The Mac Pro desktop system seems to be on shaky ground, however. Not only is it Apple’s most expensive PC, but it hasn’t been updated in two years — a lifetime for the professionals that make the most of cutting-edge processor power. The iMac is unlikely to be discontinued, although it’s long overdue for a visual redesign.

Expect Apple to continue taking a bigger chunk of PC sales in 2012, and if the MacBook Air line is expanded (or consolidated) to include pro-level 15-inch and 17-inch models, Samsung (PINK:SSNLF) and other ultrabook manufacturers might not see the sales they’re hoping for. The iPad will continue to relentlessly cannibalize PC sales (including those of Macs), but Apple isn’t ready to go all tablet any time soon.

As of this writing, Brad Moon did not own a position in any of the stocks named here.