I'm using a Postgresql 8.4 DBMS in order to store datas and other things, but I was curios to install and test the new Postgresql 9.x.
Thanks to this post I've installed very quickly Postgresql 9 to the machine, without removing the "old" Postgresql 8, which contained all the data.
In order not to stop using all the apps I needed, I changed the Postgresql 9 default port:
Rebooting the machine both Postgresql services started correctly and adding the "-p ####" or "--port=####" option at the end of pg* command I was able to connect with both DBMSes
/etc/postgresql/9.0/main/postgresql.conf:port = 5433
Eventually take a look at
pg_wrapper(postgresql-common and postgresql-client-common Ubuntu package)in order to call the correct version of the DBMS you're invoking.