I found that \usepackage{natbib} works even without dev-tex/natbib installed. It is because is included to dev-texlive/texlive-latex main package, here: /usr/share/texmf-dist/tex/latex/natbib/natbib.sty dev-tex/natbib installs this file here: /usr/share/texmf-site/tex/latex/natbib/natbib.sty ("site" instead of "dist"). So they don't conflict and can be installed at the same time. Please either remove it from the main package or delete natbib package and drop it as dependence of other package (at least dev-tex/revtex).
Andreas, you added this one as the texlive version was not new enough for your needs IIRC. I have not checked but I think it is not the case anymore. Hence, if you don't care about this pacakge anymore, please last rite it. Thanks.
+ 11 Sep 2013; Andreas K. Huettel <dilfridge@gentoo.org> package.mask: + Mask dev-tex/natbib for removal, bug 483974 + Will cloes the bug once it's gone.
Removed.