Portsentry works just fine on amd64. I added the amd64 string to the ebuild KEYWORDS for version 1.2 and then "emerge portsentry" and it built and installed with no problems. I also tested the program (all 6 modes of operation) and it seems to work just fine. I recommend adding amd64 to the ebuild file.
Tested ok. ~amd64 added to ebuild in cvs.