QT and GTK already run in Wayland. I don't know about the rest.
IT's not terrible if they don't. Everything that currently runs in X will be able to run fine on Wayland because you will have a Wayland X server.
X Windows is a protocol. This is analogous to HTTP.
X Clients are your applications. This is analogous to Web Sites.
X Server is were your applications get rendered. This is analogous to a Web browser.
So running your X Server as the manager for your hardware is like running a Web browser that fiddles with settings on your PCI bus... which is a terrible design. In a well designed system there is no need to run a X Windows server that manages your hardware. It's a network protocol after all. Do you want something that listens on your network having direct access to your PCI bus and full root access to everything in your system?
So you will be able to run X windows applications on top of Wayland the same way you run Websites on Wayland... by using a application that implements X Windows.
Just like how you can run X applications on Microsoft Windows and on OS X... Microsoft Windows can even run GLX applications.
As far as Nviida proprietary drivers (which are the only proprietary drivers worth caring about) "supporting" Wayland.. all Wayland technically needs is EGL support.
http://en.wikipedia.org/wiki/EGL_(OpenGL)
People can already run Wayland on Android's graphic stack... I don't see why it would be a great leap to run it on Nvidia's graphic stack. Nvidia needs to support EGL on Linux anyways for embedded systems. It's natural that they would be able to support it on desktop systems.
Meanwhile Nvidia proprietary driver users will be able to run Wayland by running Wayland full screen on top of NVidia/Xorg X server until Nvidia decides to support the EGL extensions Wayland needs.