gtk+-2.8.0 creates this file in /etc/gtk-2.0/i686-pc-linux-gnu/ and /etc/gtk-2.0/x86_64-pc-linux-gnu/ but many packages (like libgnomeui) look for this file in /etc/gtk-2.0/ . If you upgrade gtk+ when you have a working gnome enviroment you will get an image-less gnome enviroment, that's bad :-\
Oh, I'm using the hardmasked gnome packages (just copied the gnome packages from package.mask to package.unmask plug x11-libs/cairo).
Yeah, this is because somebody has commented out the lib64.patch line in the gtk-2.8.0 ebuild. I assume there was some problem applying the patch to this version? I'll have a look at producing an updated patch.
Updated multilib patch added to gtk+-2.8.0-r1.