Were Apple to put the x86 version of its operating system on general release, Dell would begin to manufacture Apple clones. This would put enormous pressure on the price of Apple's own computers -- something the company is naturally keen to avoid.
While this is undoubtedly true, perhaps the bigger risk to Apple is that without maintaining their traditionally tight control over the hardware/software integration, the Mac OS X user experience would be likely to suffer, and thus so would Apple's reputation for quality.
What would piss Apple off even more in such a scenario would be when software vendors were slow to adopt new hardware characteristics specific to Apple models simply because those features were unavailable in the clone market.
So it's not necessarily so much about a loss of revenue for Apple (which they could, after all, potentially make up for with some appropriate licensing scheme) as it is about a loss of control, which is, after all, something Steve Jobs obviously values very highly.