I recently needed to install the PostgreSQL database server on my Mac for development with Ruby on Rails. I ran into a few gotchas, and while I was eventually able to Google and guess my way to success, it took a while. This installation guide is as much for me, 6 months from now, as it is for others who may wish to also install Postgres.
These instructions were current as of April 22, 2009, and were executed on Mac OS X 10.5.6 with Rubygems 1.3.2 and the Postgres 8.3.7 distribution. The shelf life of these instructions is probably brief, though they should work at least as a rough guide until the next major release of Mac OS X, or of Postgres.