for DVD playback:
sudo wget http://www.medibuntu.org/sources.list.d/gutsy.list -O /etc/apt/sources.list.d/medibuntu.list
wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add - && sudo apt-get update
sudo apt-get install libdvdcss2
Supposedly, this should all be included under, Applications menu, Add/Remove, All, Ubuntu Restricted Extras. But I can't ever get that to work for me. The restricted extras do get you plenty of the other missing things - like mp3, core fonts, etc. so I always set it up as one of the first things on a new install.
Note: this is probably easier than my previous post on the subject (for earlier versions)