Per this commit by Kristian Høgsberg this evening:
We can now handle fullscreen X windows. X clients request to go fullscreen buy [sic] sending a _NET_WM_STATE client message to the root window. When that happens we call into the shell interface and asks the shell to make the surface fullscreen. The shell will then resize the window, which causes the X wm to configure the X window appropriately.Though in terms of support for X clients on Wayland, sadly XWayland hadn't made it into the X.Org Server 1.14 release that's due out in March, so it will still likely be a while before seeing the X11 client support on Wayland be widespread -- likely X.Org Server 1.15 in the September time-frame for Q4'2013 Linux distributions.
Make sure we ignore configure requests from fullscreened clients and send out the synthetic configure notify as required in that case.
Finally, inspect _NET_WM_STATE before mapping so we can handle initial fullscreen correctly.