Recently I did some work for Gnome despite KDE being my primary desktop.

After a nice chat with the Gnome team I bumped gtk+ to 2.10, which uses the new cairo-1.2.0 with many new features. The new gtk+ is hard masked to ensure proper testing of the currently ~arch ebuilds so that they can be marked stable. To get the new gtk+ you have to unmask the new cairo as well as a new glib and pango.
Despite being compatible for applications, gtk+-2.10.0 uses a new directory for the engines and modules. That means you have to rebuild them to work correctly. To inform users about this change I am using the new elog functionality in the postinstall message of gtk+. After updating you can easily rebuild the affected ebuilds using portage-utils:

emerge -va1 $(qfile -qC /usr/lib/gtk-2.0/2.[^1]* )