Install pg gem under 1.9.1

Patch pg

# get rid of the pg if you have it already; you might want to do this for ruby-pg as well
gem uninstall pg

# change to some convenient working directory
cd ~/src
svn checkout http://ruby-pg.rubyforge.org/svn
cd svn/ruby-pg/trunk/ext

# download and install the patch
curl -O http://gist.github.com/raw/215956/54166b63f9cce030232b09397934315f004fb649/pg-patch.txt
patch < pg-patch.txt

# set up an important environment variable
export ARCHFLAGS='-arch i386'

ruby extconf.rb
make
make install