Connect to an URI in postgres

I'm guessing this is a pretty basic question, but I can't figure out why:

import psycopg2
psycopg2.connect("postgresql://postgres:postgres@localhost/postgres")

Is giving the following error:

psycopg2.OperationalError: missing "=" after
"postgresql://postgres:postgres@localhost/postgres" in connection info string

Any idea? According to the docs about connection strings I believe it should work, however it only does like this:

psycopg2.connect("host=localhost user=postgres password=postgres dbname=postgres")

I'm using the latest psycopg2 version on Python2.7.3 on Ubuntu12.04