I'm not a huge fan of Microsoft or Windows Vista, and I do all my primary development on a Mac. But...
The same X-Plane code that runs on Windows Vista will run on Windows 98 SE.
In that time Apple has changed the API (introducing Carbon), the ABI twice (moving from CFM to Mach-O, and GCC3 to 4*) and the instruction set (PPC to Intel), as well as the compiler twice (MPW to X-Code, or more likely Metrowerks to X-Code, and GCC 3 to 4).
I love Apple dearly, but if I write an app and burn the source code, I wouldn't be surprisde if the binaries run on Windows 5 years from now. I wouldn't make that prediction for the Mac.
*Is it fair to call the upgrade from GCC 3 to 4 an ABI change or compiler change? Not really, they're not full ones, but besides the usual slight changes, the real problem is that GCC 4 generates run-time dependencies on shared libraries that were not available in older Mac Operating Systems. This is why X-Plane is not alone in requiring OS X 10.3.9 or higher - 10.3.9 is the oldest operating system that has the runtime for GCC 4. Applications that can run all the way back to OS X 10.2 are actually building their PowerPC executable code against a different deployment target, which requires extra makefile gymnastics.