I disagree with most of the postings here.
I suspect that at some point in the future, we will see something along the line of "OS X for PC's" sold by Apple.
I realize that it would be difficult or impossible for Apple to support software sold as such across the bewildering array of hardware marketed for PCs. However, it could be "bundled" with an approved motherboard (suitable for installation in a PC case) tested and known to work with OS X, with a recommended list of graphics cards, as well. Just assemble the rest yourself, and you should be good to go.
I see this as a way for Apple to retain the high-end content creation markets (folks who would normally be using Mac Pro's) in the event that they decide it isn't worth it to keep the Mac Pro in production. Hasn't there been speculation of this of late?
There will be replies to this post to the effect that "Apple has gone the clone route before, and it didn't work for them, was harmful to them". Correct, because when they tried cloning in the 1990's (I owned a couple of SuperMac clones), they discovered that the cloners could produce more powerful products at lower prices than Apple was willing to offer. That's basic economics -- the better product gets more sales.
But if Apple is indeed considering discontinuation of the Mac Pro, it will leave a hole in the product line that others can fill. Apple can choose to collect hardware and software license fees (probably substantial) while leaving the actual design R&D and production to others.
Apple has changed directions suddenly before. Only six+ years ago, there were rumors flying that Apple had been secretly developing a version of the Mac OS that ran on -Intel- processors! Outrageous, it seemed. And Apple RESOUNDINGLY DENIED the existence of any such OS.
Yet a year later, surprise, SURPRISE! Apple announced they were abandoning the PPC architecture and moving to the Intel platform. But even more astonishing, Apple revealed that THEY ALREADY HAD a version of the OS that was up and running on Intel, and made developer hardware platforms available, as well! There aren't many around here who personally remember Watergate, but it was the equivalent of Ron Ziegler going before the press and stating that all his previous comments were
"inoperative"
OS X for PC's? Not guaranteed, but certainly possible
.