The current experimental gtk+ ebuild ignores USE-flag "cairo". At least for me it runs perfectly on 4 different machines (all x86 - Intel and Athlon with and without 3D capable grafic chips). I think --with-cairo should be added while building gtk+ on cairo-enabled systems. Maybe add a USE-flag "gtk-cairo" if appropriated. Reproducible: Always Steps to Reproduce:
[Edited] My fault, it's GNOME 2.10 but gtk+ 2.8 - corrected title
Cairo is no longer a use flag option as of 2.8, as gtk+ now depends on it.
steev is correct. cairo is now a requirement of the GTK+ 2.8 branch. It's not optional anymore.