I am running Ubuntu 12.10 and, if possible, I would like to get the list of configuration files available for a given package that is already installed.
I would prefer to do this using dpkg or apt if possible, but no problem if other tools such as aptitude or dselect are needed/recommended.
There's no need to use anything other than cat, if I understood your intention correctly:
cat /var/lib/dpkg/info/<package>.conffiles
should give you what you're after. For instance for package zsh:
% cat /var/lib/dpkg/info/zsh.conffiles
/etc/zsh/zlogin
/etc/zsh/zlogout
/etc/zsh/zprofile
/etc/zsh/zshenv
/etc/zsh/zshrc
/etc/zsh/newuser.zshrc.recommended
Regarding a case where there's no such file for a given package - it's up to the package maintainer to designate certain files as configuration. If this hasn't been done properly, you should file a bug where appropriate.
In such cases you have a couple of options.
List files belonging to the package that are in /etc/:
dpkg -L package | grep '/etc'
Fetch and inspect the source package to find out how it was compiled (which should also show you where it expects its configuration files to be located).
apt-get source package
less package-x.y.z/debian/rules
Look up the upstream project page to find documentation.
