I'll post about X-Plane 10.40 next week - but just a quick note: apparently the Rift will ship Windows-only first:
Our development for OS X and Linux has been paused in order to focus on delivering a high quality consumer-level VR experience at launch across hardware, software, and content on Windows. We want to get back to development for OS X and Linux but we don’t have a timeline.
That's from a much longer blog post describing the Rift's hardware requirements and the difficulty of moving that many pixels at that high of a framerate with ultra-low latency. (The tl;dr version is that if you have a Windows laptop you probably won't be able to run the Rift on it.)
I don't know what the failure mode will be for not meeting requirements, e.g. will the Rift simply not run, or will it do the best it can with degraded performance.
To state the obvious, if there isn't an Oculus Rift SDK and driver for OS X or Linux, then X-Plane can't support those operating systems.