If I understand Wayland correctly, this isn't something that's built into the kernel. If so, then that means that, even when Wayland becomes the default display manager, we can do a commandline install and install X.org instead of Wayland.
In that case, will X.org always be available in the repositories in case someone needs the core features of X.org?
First of all, before we jump the gun here, X.org is not going anywhere just yet.
For situations where wayland is not supported, X will remain the fallback. Plus, X will still be maintained and available.
That covered, wayland will provide many of the features X does, can run a nested X server, etc.
Sorry for not including links, I might do that if I update my answer later.
It's hard to predict what things will be like 10 years out, so "always" is a pretty strong word. :-)
However, I can safely make the following forecast:
a) Ubuntu won't be switching away from X.org in the short term. Certainly not for natty, and likely not natty+1 either. The exact schedule is doing to depend a lot on how quickly a lot of development work on compiz, unity, gtk and other bits can get done.
b) Even when we do switch, we'll be keeping X.org available for legacy applications. I don't know how long we'll keep it, but likely for several years at least.
c) Some drivers (like -fglrx and -nvidia) won't work with Wayland. There's also unusual graphics hardware, virtualization environments, and so on that won't meet Wayland's requirements for some time. So it's likely some "retro mode" that runs X just like today will be available, for users who prefer not to use Wayland.
d) Upstream X.org developers are supportive of moving the ecosystem to something Wayland-based. I would predict that eventually upstream will simply stop developing/supporting X.org, and that will be what brings X.org to an end. When will that happen? 10 years out? Maybe, I don't know.
In any case, don't panic. X.org isn't going away any time soon. Wayland is still in diapers.
My understanding is that several "xclients" are still not available for wayland. And then there is is this announcement by nvidia that they have no plans of supporting Wayland. i have to agree with @Roland here that the migration is not going to happen in the near future.
