By enable DVD playback,you can play encrypted DVDs with Mplayer,xine,VLC media player,and so forth.
But DVD playback is not enabled in ubuntu by default since it could result in some legal issues with Hollywood types.
It is very easy to enable it in ubuntu.Just need two commands,and not need to add third party repositories.

You just need to run the commands below fit for your version of ubuntu.
For ubuntu 9.04:

sudo apt-get install libdvdread4
sudo /usr/share/doc/libdvdread4/

For ubuntu 8.10:

sudo apt-get install libdvdread3 gstreamer0.10-plugins-ugly
sudo /usr/share/doc/libdvdread3/

For ubuntu 8.04 (i386):

sudo apt-get install libdvdread3
sudo /usr/share/doc/libdvdread3/

For ubuntu 8.04(amd64):

sudo apt-get install build-essential debhelper fakeroot
sudo /usr/share/doc/libdvdread3/