In my Ubuntu 9.10 "Karmic Koala" installation, I wasn't able to watch a DVD, even though libdvdcss was installed. The player windows just closed themselves immediately after starting playback. After some searching, I found the solution.
To solve that problem, first install the package ubuntu-restricted-extras from the universe repository...
$ sudo apt-get install ubuntu-restricted-extras
...and then execute the following script.
$ sudo /usr/share/doc/libdvdread4/install-css.sh