In order to play some commercial DVDs in Ubuntu, you have to install libdvdcss.
just run these commands in a terminal:
sudo apt-get install ubuntu-restricted-extras
That's it, enjoy your movies!!!
It works in Ubuntu 10.04 and 9.10.
ubuntu-restricted-extras package installs various audio codecs (including MP3 playback/encoding support), Microsoft fonts, Java runtime, Flash plugin and Lame.
If DVD playback is the only feature you need, you can just install libdvdread with this command:
sudo apt-get install libdvdread4