Please add "emacs? ( app-emacs/ruby-mode )" to PDEPEND of ruby, as already suggested earlier in bug #156388.
I also want to add a similar USE flag for xemacs, pulling in app-xemacs/ruby-modes). However, this breaks for arm, hppa, mips, sh, sparc-fbsd and x86-fbsd since they don't have a version of app-xemacs/ruby-modes keyworded. Judging from the xemacs keywords I think that hppa would like to keyword app-xemacs/ruby-modes while the other arches would want to put dev-lang/ruby:xemacs in their use mask. I've removed your keywords from the new dev-lang/ruby-1.8.6_p110-r2 ebuild for now. Please resolve the keyword issue and add back your keywords, or leave a note on the bug and I'd be happy to resolve the issue for you. @sparc-fbsd: note that you also have not keyworded net-proxy/dante, which gets pulled in through the optional socks5 USE flag.
app-xemacs/{debug,ruby-modes} marked ~hppa.
I've add xemacs to package.use.mask for ruby on arm, mips and sh, as I'm doing a security bump on ruby.
This bug can now be closed as recent ebuilds have both the emacs and xemacs USE flags and repoman full doesn't complain anymore due to Richard's action.