gtkmm 2.9 wraps new API in GTK+ 2.9, and is API/ABI-compatibile with gtkmm 2.8, 2.6 and 2.4. It is a version of the gtkmm-2.4 API. This will become the API-stable gtkmm 2.10, wrapping GTK+ 2.10.

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

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