go to gtkmm's website and click on news, they are still properly maintained, they apparently are caught up to gtk 3.0 or so.
there are several current programs that use gtkmm as there gui api. and im pretty sure its a requirement to keep maintained as part of the gnome project.
honestly, its always annoying when people compare gtk to qt based on how easy the api is. its not exactly fair to compare a c++ (with there awkward preprocessor..) api to a c api that basically uses preprocessor hacks to achieve a object oriented feel.
gtkmm uses as much standard c++ libraries as it can, like the stl.