I\'ve installed R3.0 on my Mountain Lion Mac and after some trial and error the version of GTK suggested by http://r.research.att.com/ for R3.0: GTK+ 2.24.17 framework.
In addition to jverzani's answer, if you want to use Homebrew:
brew install gtk+ export PKG_CONFIG_PATH=/usr/X11/lib/pkgconfig:$PKG_CONFIG_PATH
and then install RGtk2 and cairoDevice from source.