From a design point of view, the other poster had it right. It was being put simply so that more people could understand what it meant. However, they were probably referring to the 9X lines of Windows when DOS really still was the basic core of it all. Darwin is not a subsystem - Apple likes to explain it as one because it really makes many Mac fans more comfortable, I guess, but Darwin is really the very core of the OS. This core is available on x86 machines, but then you're stuck with X11 which is a really pathetic, in my opinion, GUI system.
Aqua is only a GUI, very much like Windows 95 was. Aqua just happens to be a lot -better- than any Windows GUI. You can run a different GUI interface if you want to, apparently with and without using Aqua. The reason Aqua isn't being ported to X86 and sold is very simple: Apple would go broke.
Apple survives by providing compelling reasons to purchase Apple hardware. They then hit you up -again- for the software. I wish Apple would make the software free, but continue to charge for the hardware. But, this probably makes almost as poor sense to them financially as selling an x86 version does.
Now why does the idea of selling an x86 platform of Aqua cost Apple money? Because the interest in OS X, even with a 20% market share, wouldn't even come close to being competitive with Microsoft, and the support costs they'd incur by running with your average PC hardware would be absolutely monstrous. What's more likely being considered is that Apple would start building its own hardware based on x86 processors, but continue to only have Apple operating systems that run on Apple hardware. My hope is actually that Apple will be using the new IBM Power 4 chip, and in enough quantity that the prices can come down a bit.
And actually, the machine and hardware itself -is- still a very strong selling point for Apple. Not necessarily because it's faster (it isn't) but because it is a better design and is assembled with higher apparent quality. And time after time, people beat the dead horse that Apple is more expensive but entirely forget the fact they'd probably only be about 10% under the Apple cost -before- having to buy an OS and software applications equivalent to what Apple includes with OS X. Apple makes a quality machine, especially compared to the hundreds of really crappy PC builders out there.