Gentoo traditionally lags a bit behind in releases of the Gnome Desktop.
And now as we near the release of Gnome 2.20, Gentoo is right on time (read: late) with their release of Gnome 2.18. I got this to go very smoothly with the use of this:
emerge -C control-center && emerge -uD world || emerge sys-devel/gettext dev-perl/XML-Parser && revdep-rebuild -X && emerge -uD world
Remove the control-center package as it blocks a package in the new version of Gnome, upgrading all my packages, rebuilding parts of the system to account for expat breakage during the upgrade, and finally finishing off the upgrade after it breaks.
Then log out. (restarts xorg) And back into your Gnome goodness....