I do: # emerge setiathome It says: !!! all ebuilds that could satisfy "setiathome" have been masked. Just so you know: There is an x86_64-pc-linux-gnu binary on the setiathome website, so I assume it could be unmasked. SETI downloads page at http://setiathome.berkeley.edu/unix.html
AMD64 support was added for setiathome on 13 Oct 2003. Still marked as unstable (~amd64).