There's been experimental work on bringing the GNOME Shell and Mutter to Wayland/Weston, but nothing that's been merged yet and ready for GNOME desktop users. The GTK3 tool-kit does have Wayland support and it's continuing to be improved. Rather than GNOME jumping on the Mir bandwagon, they want to move now at a vicious rate in supporting Wayland.
Red Hat's Matthias Clasen took a proposal on Tuesday to the GNOME release team. Matthias and several other GNOME developers met with Kristian last week to talk about GNOME plans for Wayland. "So far, we've silently assumed that Wayland is the future display system on Linux, and that we will get to using it eventually...The recent Mir announcement makes it a bit more urgent that we put our weight behind Wayland and help it reach its full potential. Doing this also fits our mandate from last Guadec to set stronger technical directions."
Finishing up GNOME support for Wayland is to big to get done in one development cycle but Clasen proposes that it be completed over two development cycles. "By the fall, have gnome-shell optionally work as a Wayland compositor, and have the gtk backend substantially complete - by next spring, have a complete port of all desktop functionality. Making applications run natively can happen independently at its own speed - we will support X clients transparently."
This would mean that by spring of 2014, GNOME 3.12 will be a complete port of GNOME to Wayland. Compatibility for most GNOME components will be maintained to run under X while all core GNOME components will work under Wayland. The goal for GNOME 3.10 this fall is for the GNOME Shell to optionally work as a Wayland compositor, the GTK+ tool-kit being good enough for day-to-day use, and applications can transparently fall-back to using XWayland.
More details on this proposal can be found on the GNOME Live Wiki and the release-team mailing list. In response, there's even talk of GNOME 4.0 being exclusive to Wayland.