Ubuntu Server 9.04 comes with PostgreSQL 8.3 installed. This is working correctly; I am able to log in, list the databases, etc.
I have tried to follow the instructions here for installing PostGIS, including a sudo apt-get install postgis. This appeared to install version 1.3 of PostGIS okay, but then doing a locate lwpostgis.sql or locate postgis didn't return any results. Since I was unable to locate these .sql files, I was unable to continue with the installation guide.