gtkmm 2.5 wraps new API in GTK+ 2.5, and is API/ABI-compatibile with gtkmm 2.4. The new API is unstable, until this become the API/ABI-stable gtkmm 2.6.

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