update after adding `for-science` should remove the need to install the packages twice. also fixed typo.

gistfile1.txt

#!/bin/bash
#
# Install Postgres 9.1, PostGIS 2.0 and pgRouting on a clean Ubuntu 12.04 install (64 bit)
# updated to PostGIS 2.0.1

# basics
apt-get install python-software-properties
apt-add-repository ppa:sharpie/for-science  # To get GEOS 3.3.3
apt-get update

# install the following packages
apt-get -y install postgresql-9.1 postgresql-server-dev-9.1 postgresql-contrib-9.1 gdal-bin binutils libgeos-c1 libgeos-dev libgdal1-dev libxml2 libxml2-dev libxml2-dev checkinstall libproj-dev libpq-dev build-essential

# fetch, compile and install PostGIS
wget http://postgis.refractions.net/download/postgis-2.0.2.tar.gz
tar zxvf postgis-2.0.2.tar.gz && cd postgis-2.0.2
./configure --with-raster --with-topology
make    
make install

## pgrouting - Add pgRouting launchpad repository
sudo add-apt-repository ppa:georepublic/pgrouting
sudo apt-get update

# Install pgRouting packages
sudo apt-get install gaul-devel \
        postgresql-9.1-pgrouting \
        postgresql-9.1-pgrouting-dd \
        postgresql-9.1-pgrouting-tsp

# Install osm2pgrouting package
sudo apt-get install osm2pgrouting

# Install workshop material (optional)
sudo apt-get install pgrouting-workshop

# create a table called 'routing'
sudo su postgres
createdb routing

# add PostGIS functions (version 2.x) to 'routing'
psql -d routing -c "CREATE EXTENSION postgis;"

# add pgRouting core functions to 'routing'
psql -d routing -f /usr/share/postlbs/routing_core.sql
psql -d routing -f /usr/share/postlbs/routing_core_wrappers.sql
psql -d routing -f /usr/share/postlbs/routing_topology.sql