Despite my system having gtk+-2.6.8 installed, gtk-engines install .so and .la fiels into /usr/lib/gtk-2.0/2.4.0/engines, instead of /usr/lib/gtk-2.0/2.6.0/engines, despite the fact that GTK2_ENGINES_DIR gets set properly when gtk2-engines.eclass gets included. Is that on purpose? Both gtk-engines-2.6.3 and -2.6.2 are doing this. I also have word from a friend that gtk-qt-engine ebuild behaves like this.
It gets installed there because 2.4.0 is the earliest version of Gtk+ with backwards binary compatibility. gtk-engines gets that version from 'pkg-config --variable=gtk_binary_version gtk+-2.0' and that's the expected behaviour.