I have just finished installing Ubuntu 9.04 on a Gateway laptop, everything worked well from video to wireless until I popped in a DVD to watch. Did the updates and still nothing, until I ran across this command on the Internet, tried it and everything is golden now.
sudo apt-get install ubuntu-restricted-extras totem-xine libxine1-ffmpeg libdvdread4
The explanation when the commands are needed are printed below:
The movie players provided in Ubuntu can play back unencrypted DVDs. However, many commercial DVDs are encrypted with a weak algorithm called Content Scrambling System (CSS). You can enable playback of encrypted DVDs with MPlayer, xine and Totem-xine by installing libdvdread4.
The CSS key sets are licensed to manufacturers who incorporate them into products such as DVD drives, DVD players and DVD movie releases. Most DVD players are equipped with a CSS Decryption module.