In early 00's Apple actually provided x86 binaries for the Darwin (in form of essentially complete rpm-based distribution with Darwin kernel). Obviosly this didn't include the high-level NeXT/MacOS X ObjectiveC frameworks.
As a shameless plug, the Darwin 0.1/0.3 sources which align with OS X Server 1.0 & 1.2 actually still compiled on intel just fine. Even better you could take a Rhapsody DR2 system, and overlay the newer stuff on top of it, and it'd still happily run.
And Rhapsody (Mac OS X Server 1.0) shipped a developer preview on Intel, including UI (this was pre-Aqua and Mach 3.0, but otherwise a lot of similarities to MacOS X 10.0).
