For being a feature to GTK+ 3.10 when released in September, client side decorations (CSD) are now ready for Wayland. The initial Wayland CSD work for GTK+ was done by Kristian Høgsberg and Rob Bradford at Intel. The initial commit can be seen here.
A follow-on commit added support for enabling GTK+ client-side decorations on non-Wayland platforms. This CSD support can be turned on for other platforms via the GTK_CSD environment variable.