Most commercial DVDs are encrypted with CSS (the Content Scrambling System), which attempts to restrict the software that can play a DVD. By installing the libdvdcss2 package you can play encrypted DVDs with MPlayer.
Installing libdvdcss in Ubuntu 10.04
Install the libdvdread4 package (no need to add third party repositories) via Synaptic or command line:
sudo apt-get install libdvdread4
Then open a terminal window and execute:
Rebooting may be necessary.
Legal Warning: Check with your local laws to make sure usage of libdvdcss2 would be legal in your area.