Update: couchdb svn r559 changed the behavior of the --with-erlang configure call. The ebuild has been updated.
Since Noah went and modernized the build system, rendering my three-line ebuild obsolete, I bumped it a bit. I had to tweak some of the local-state info to fit with the gentoo structure and why are permissions always so annoying?
Anyway: couchdb-9999.ebuild. The ebuild's md5sum is 91c35549d1c2773ba583a4c518c960ca.
I have an 0.6.3 ebuild too, but since so much has changed since then I doubt many would find it useful.