gtkmm 2.11 wraps new API in GTK+ 2.11, and will become stable gtkmm 2.12 when GTK+ becomes GTK+ 2.12. It will be API/ABI-compatibile with gtkmm 2.10. 2.8, 2.6 and 2.4. It is a version of the gtkmm-2.4 API.

gtkmm stays in-sync with GTK+ by following the official GNOME release chedule:
http://www.gnome.org/start/unstable/

Bindings for the rest of the GNOME Platform are also available, and are also API-stable.

http://www.gtkmm.org