portage-2.1.8 has new --rebuilt-binaries option

In portage-2.1.8 there's a new emerge --rebuilt-binaries
option that is very useful for people who build binary packages and
install them on multiple computers (using PORTAGE_BINHOST or shared
PKGDIR). The option causes packages to be automatically reinstalled in
cases when rebuilt binary packages are available (due to revdep-rebuild
or similar cases).
Rebuilds are detected by comparison of BUILD_TIME package metadata.
This option is enabled automatically when using binary packages
(--usepkgonly or --getbinpkgonly) together with --update and --deep.

http://blogs.gentoo.org/zmedico/2010/03/03/rebuilt_binaries_portage_2_1-8