The current ~amd64 keyworded libpq-8.1.5 and postgres-8.1.5 ebuilds are blocking each others' older versions (<=8.1.4), thus making an incremental upgrade of either impossible - if one wants to upgrade libpq, it is blocked by postgres, and vice versa. According to upstream's release notes, they haven't broken any compatibility between versions 8.1.4 and 8.1.5, so *both* of these blocks should be lifted: http://www.postgresql.org/docs/8.1/static/release.html#RELEASE-8-1-5
Oops, that's postgresql and not postgres.
*** This bug has been marked as a duplicate of 154779 ***