By default ubuntu (9.10 and 10.04) Ubuntu does not come with the needed codecs to play dvds for legal reasons. It will only take you a few minutes to make the needed and change. First you will need to open a terminal window and issue the following commands
Click on Applications > Accessories > Terminal
Type or copy and paste the following command in the terminal window:
sudo apt-get install ubuntu-restricted-extras
Note: About the Install of ubuntu-restricted-extras
Description: Commonly used restricted packages for Ubuntu This package depends on some commonly used packages in the Ubuntu multiverse repository.
Installing this package will pull in support for MP3 playback and decoding, support for various other audio formats (GStreamer plugins), Microsoft fonts, Java runtime environment, Flash plugin, LAME (to create compressed audio files), and DVD playback.