I believe this was one of the talks at Postgres Open - I came across the slides from most of the others earlier at http://wiki.postgresql.org/wiki/Postgres_Open_Talks_2011, and they have some good tips on scaling with Postgres.

There was one interesting tidbit in one of the Heroku presentations : they apparently run over 150,000 databases ( https://wal-e-pgopen2011.herokuapp.com/#6 ) which is incredible.