perldiver

This page explains it really well:  http://developer.apple.com/internet/opensource/postgres.html

Short form:

1. Install Fink (you're going to want Fink and probably FinkCommander anyway, if you are a developer)

2. sudo  /sw/bin/fink install readline

3. Download and unzip the latest tarball of the Postrgres code from ftp://ftp12.us.postgresql.org/postgresql/latest/ (or another mirror)
4. Configure with the options of your choice.  These were my choices:
1. ./configure --with-includes=/sw/include/ --with-libraries=/sw/lib --with-perl --with-python --with-bonjour --enable-thread-safety
   
5. Build
1. make
2. make install

Ta-da, you're done!  Pg is now built on your OSX machine and ready to be initialized. 

For completeness, here's the initialization procedure as well:

1. Create a postgres user
2. sudo bash
3. mkdir -p /usr/local/pgsql/data
4. chown -R postgres /usr/local/pgsql/data
5. su postgres

6. /usr/local/pgsql/bin/initdb -D /usr/local/pgsql/data

7. /usr/local/pgsql/bin/psql
   
8. CREATE USER your_user_name WITH PASSWORD 'fooBar';
9. CREATE DATABASE your_user_name;

That's it.  You may want to add some or all of the following to a .profile, .bashrc, or other file that gets run at login, just to save yourself some typing or make link files available, but it shouldn't be strictly necessary: 


export LD_LIBRARY_PATH=/usr/local/pgsql/lib
export MANPATH=/usr/local/pgsql/man:$MANPATH
export PATH=/usr/local/pgsql/bin:$PATH