IIRC when introducing switch to Intel processors Jobs told that each version OS X was running on Intel since day one.
How about we chalk that up to marketing. I don't think the original Mac OS X ran on X86 nor would I be inclined to believe it.
You can see the timeline here: http://en.wikipedia.org/wiki/Rhapsody_%28operating_system%29
Rhapsody was the first OSX.