I have version 3.0.1 running on Ubuntu 8.04.
When I create a new user from within xTuple, the user appears in the usr table properly, and in the Postgresql user table (\du in psql shows the new user.)
However, the new user is not assigned the password in the Postgresql user table and thus, later, the connection simple does not work.
Doing an ALTER USER name SET ENCRYPTED PASSWORD 'blah'; fixed the problem.
Note: with an existing user, the password will not be changed and that works "nicely", except that the password won't necessarily match.