For the impatient

% /usr/bin/ruby -e "$(curl -fsSL https://raw.github.com/gist/323731)"
% brew install postgresql
% initdb -E utf8 -D /usr/local/var/postgres
% cp /usr/local/Cellar/postgresql/9.0.4/org.postgresql.postgres.plist ~/Library/LaunchAgents/
% launchctl load -w ~/Library/LaunchAgents/org.postgresql.postgres.plist
% psql -d postgres -f /usr/local/Cellar/postgresql/9.0.4/share/postgresql/contrib/adminpack.sql
% brew install proj
% brew install geos %you should really read below before running this
% brew install postgis or brew install --HEAD postgis to install postgis2 from svn
% createdb -E utf8 template_postgis
% psql -d template_postgis -f /usr/local/Cellar/postgis/1.5.3/share/postgis/postgis.sql
% psql -d template_postgis -f /usr/local/Cellar/postgis/1.5.3/share/postgis/spatial_ref_sys.sql

1. Install homebrew

%open https://github.com/mxcl/homebrew
%open https://github.com/mxcl/homebrew/wiki/Installation
%/usr/bin/ruby -e "$(curl -fsSL https://raw.github.com/gist/323731)"

2. Install Postgres

%brew install postgresql
....
If builds of PostgreSQL 9 are failing and you have version 8.x installed,
you may need to remove the previous version first. See:
  https://github.com/mxcl/homebrew/issues/issue/2510
 
To build plpython against a specific Python, set PYTHON prior to brewing:
  PYTHON=/usr/local/bin/python  brew install postgresql
See:
  http://www.postgresql.org/docs/9.0/static/install-procedure.html
 
 
If this is your first install, create a database with:
  initdb /usr/local/var/postgres
 
If this is your first install, automatically load on login with:
  mkdir -p ~/Library/LaunchAgents
  cp /usr/local/Cellar/postgresql/9.0.4/org.postgresql.postgres.plist ~/Library/LaunchAgents/
  launchctl load -w ~/Library/LaunchAgents/org.postgresql.postgres.plist
 
If this is an upgrade and you already have the org.postgresql.postgres.plist loaded:
  launchctl unload -w ~/Library/LaunchAgents/org.postgresql.postgres.plist
  cp /usr/local/Cellar/postgresql/9.0.4/org.postgresql.postgres.plist ~/Library/LaunchAgents/
  launchctl load -w ~/Library/LaunchAgents/org.postgresql.postgres.plist
 
Or start manually with:
  pg_ctl -D /usr/local/var/postgres -l /usr/local/var/postgres/server.log start
 
And stop with:
  pg_ctl -D /usr/local/var/postgres stop -s -m fast
 
 
Some machines may require provisioning of shared memory:
  http://www.postgresql.org/docs/current/static/kernel-resources.html%SYSVIPC
 
If you want to install the postgres gem, including ARCHFLAGS is recommended:
    env ARCHFLAGS="-arch x86_64" gem install pg
 
To install gems without sudo, see the Homebrew wiki.
==> Summary
/usr/local/Cellar/postgresql/9.0.4: 2577 files, 35M, built in 3.1 minutes
brew install postgresql  188.73s user 62.38s system 106% cpu 3:55.06 total

3. Postgres Post Installation. Initialize DB.

% initdb -E utf8 -D /usr/local/var/postgres
The files belonging to this database system will be owned by user "rupert".
This user must also own the server process.
 
The database cluster will be initialized with locale en_AU.UTF-8.
The default text search configuration will be set to "english".
 
creating directory /usr/local/var/postgres ... ok
creating subdirectories ... ok
selecting default max_connections ... 20
selecting default shared_buffers ... 2400kB
creating configuration files ... ok
creating template1 database in /usr/local/var/postgres/base/1 ... ok
initializing pg_authid ... ok
initializing dependencies ... ok
creating system views ... ok
loading system objects' descriptions ... ok
creating conversions ... ok
creating dictionaries ... ok
setting privileges on built-in objects ... ok
creating information schema ... ok
loading PL/pgSQL server-side language ... ok
vacuuming database template1 ... ok
copying template1 to template0 ... ok
copying template1 to postgres ... ok
 
WARNING: enabling "trust" authentication for local connections
You can change this by editing pg_hba.conf or using the -A option the
next time you run initdb.
 
Success. You can now start the database server using:
 
    postgres -D /usr/local/var/postgres
or
    pg_ctl -D /usr/local/var/postgres -l logfile start

4. Postgres Startup

% cp /usr/local/Cellar/postgresql/9.0.4/org.postgresql.postgres.plist ~/Library/LaunchAgents/
% launchctl load -w ~/Library/LaunchAgents/org.postgresql.postgres.plist
%telnet 127.0.0.1 5432
Trying 127.0.0.1...
Connected to rupert-mbp.
Escape character is '^]'.
% psql -d postgres -f /usr/local/Cellar/postgresql/9.0.4/share/postgresql/contrib/adminpack.sql

5. Postgis
Note this will install dependencies, PROJ4 and GEOS. At the time of writing this, we have a problem with GEOS. Need to update GEOS formula to 3.3.1. For more info read:

% open https://github.com/mxcl/homebrew/issues/8151
% open https://gist.github.com/1306088
% brew edit geos
% brew install proj
% brew install geos
% brew install postgis OR brew install --HEAD postgis
% createdb -E utf8 template_postgis
% psql -d template_postgis -f /usr/local/Cellar/postgis/1.5.3/share/postgis/postgis.sql
% psql -d template_postgis -f /usr/local/Cellar/postgis/1.5.3/share/postgis/spatial_ref_sys.sql

6. osm2pgsql

~% brew install osm2pgsql
==> Checking out http://svn.openstreetmap.org/applications/utils/export/osm2pgsql/
==> ./autogen.sh
==> ./configure
==> make
/usr/local/Cellar/osm2pgsql/HEAD: 6 files, 328K, built in 70 seconds

Ok.