gtkmm 2.6 wraps new API in GTK+ 2.6, and is API/ABI-compatibile with gtkmm 2.4.

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.