gtkmm provides a C++ interface to GTK+. gtkmm 2.4 wraps additional API in GTK+ 2.4. gtkmm 2.4 installs in parallel with gtkmm 2.2, so you can have both installed at the same time. glibmm is now a separate module, for use in non-GUI software.

gtkmm stays in-sync with GTK+ by following the official GNOME Platform Bindings release schedule:
http://www.gnome.org/start/2.5/bindings/

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