One of the independent developers that has been involved with the ongoing Wayland/Weston work for some time has posted a video and information about running various GTK applications under Wayland with an Ubuntu 12.04 LTS host. Additionally, he's written an unofficial "State of Wayland" report as of early April.
First of all, to get any GTK applications running under Wayland on Ubuntu 12.04 he had to rebuild the GTK library with the Wayland back-end support, since GTK for Wayland in Ubuntu 12.04 has big issues at the moment. However, in terms of those Wayland-GTK problems, they're being sorted out and should be in good shape for the next release (though that's out of the question for Ubuntu 12.04).
With the re-built GTK for Wayland, the developer got the following GTK applications working: vte, gnome-calculator, file-roller, charmap, gnome-sudoku, gwibber, transmission-gtk, brasero, gnome-sound-recorder, and baobab. It's not the most comprehensive range of applications to allow for immediately migrating from an X environment to a native Wayland setup, but it's a start.
One of the tested applications that failed to work was the Google Chromium web-browser, though others have got it running under Wayland as there's long been work on Chrome/Chromium for Wayland and Google also wants Wayland for Chrome OS. Though Chromium within Ubuntu 12.04 can work with the X output.
Other GTK applications failing to work right now with Wayland using the native display output was Nautilus, Gedit, Mahjongg, Evince, Shotwell, Empathy, Totem, Rhythmbox, and the GNOME System Monitor.
This GTK application support state for Wayland was shared on the Wayland mailing list. Also shared was a video of some GTK applications running (and some failing) on Wayland. That Ogg Video is also embedded below for convenience.
He then went on to write the unofficial State of Wayland via the wayland-devel mailing list. He summarized that the open-source Intel/Radeon/Nouveau drivers are working, XWayland is making progress, Wayland usability is limited to applications migrating to GTK3 and QT5, EFL and Clutter have complete support, the web-browser support is a ways behind, weston-terminal works okay as a terminal, and there's various open bugs.
If you missed it from the Linux Foundation Collaboration Summit last week, be sure to read (and watch the videos) about what the Intel OTC developers were saying about Wayland/Weston. If you're wanting to try out Wayland with the reference Weston compositor yourself, one easy option is to use Rebecca Black.