It would be nice to have a package set that would make Portage check the checksums of all locally installed packages against the ones on the binhost and select all packages with a different checksum.
We've had the --rebuild-binaries option and @rebuit-binaries set for some time now: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=744b5b12943ba4dbe27a694250c8a5d390b6e5bc http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=b4a4c9af72e036d8a0b2698c823cdf2979315369