There are two issues in paragraph 3.2.1. PostgreSQL of the documentation:
It looks like a specific version of PostgreSQL is required (8.X) which is not the default version of PostgreSQL on Ubuntu 12.04 LTS (9.1). Actually the default Ubuntu package is OK.
You need to install the three following packages: postgresql-8.X, postgresql-client-8.X, and postgresql-plpython-8.X.`.
It refers to the PostgreSQL project online documentation but the link points to the PosgreSQL Web site which is rather vague. Some of our users went to the PostgreSQL Web site, hit Download|Ubuntu and from there got the impression they need alternative repositories and versions. CubicWeb requirements and installation process needs to be detailed with steps simple enough for junior sysadmins. You could as well omit the link to the PostgreSQL Web site.