It's an irresistible rumor because there's a certain logic to it (for those of us who don't use Macs) and because it's impossible to disprove. Nonetheless, it makes little to no sense for Apple to pursue such a strategy.
The most important outcome to any such strategy would be the certain destruction of Apple's hardware business. Make no mistake about it: Pretty and slick as Apple's systems may be, on a level playing field with cheap and ugly white boxes that cost hundreds or thousands less, they will lose. Apple surely knows this. Therefore, the release of an x86 version of Mac OS would need to be part of a larger plan to move to a software-focused company, a la Microsoft.
Put it that way ("be like Microsoft") and the idea becomes appealing again at some level, but the time for this strategy has come and gone. Had Apple pursued this strategy in the late 80s or early 90s--when 80386 and 80486 PCs were able to run a Macintosh-class operating system, and PC operating systems were less compelling--the market would have been Apple's for the taking.
Indeed, in the enterprise--where Macs are rare outside of design or marketing departments--some feel a PC architecture version of the Mac OS would now be more desirable. I really don't see why.
The reason Macs didn't appeal to enterprises 10 or 15 years ago--namely, lack of support for important hardware and software--is much less of an issue today. The big issue today would be applications, especially in-house applications.
You would probably have application problems, especially with in-house applications. Add to this the fact that Mac applications would have to be ported from PowerPC to x86. Will there be a Microsoft Office for Mac OS/x86? Tough odds to set on that one. Still, most shops can overcome the application issues, especially using a platform that's relatively portable, like Java.
But why would you go through such a major rip and replace? If you hate Windows so much and your company is willing to disrupt your users, you have always had PC-based alternatives. And if Macs are so compelling, why not just buy the ones they sell now? The cost difference compared to standard PCs is not trivial, and neither is the cost of the disruption involved in moving large numbers of users from Windows to the Mac.
In creating a Mac OS for the PC platform, Apple's most significant problem would be the porting of device drivers. Apple could try to port their device driver model from Mac hardware (over which they have complete control) to the PC architecture (which nobody controls), but it would take a lot of work to port all those drivers. (If anyone controls the PC architecture it's Microsoft, and that's no help for Apple.)
And that doesn't address all the PC devices that don't work on the Mac. Hundreds of device drivers would need to be written for PC-only devices. Indeed, it was this fact that discouraged Apple when the company first considered a PC port of Mac OS about ten years ago (project "Star Trek"). A friend who was involved in the project told me at the time that the scale of the device driver work that needed to be done was what killed the idea, and that job has only increased in size in the interim. Think about all the Winmodems, multifunction fax/printers, and uninterruptible power supplies out there. Apple owns none of this device driver code and generally they would have to convince the third parties to write it. So if there is going to be a Mac OS/x86 don't assume you can install it on just any PC you have. (You can't even do that with Windows XP.)
And remind me again why replacing one proprietary, closed-source operating system with another is worth doing? I ask this not because I believe deeply in open-source operating systems, but because there are people who do, and I don't imagine them getting pumped up about giving Apple more power. You might admire Apple or Microsoft for their products, but neither is a cause worth cheering for.
So if you're looking ahead to the day when your users will be running Macs, stop waiting for the PC version. Apple's not likely to destroy their hardware business just so that you can spend less money on their hardware. Go out and buy Macs if they're really that much better.