This information is scattered about the web but I decided to collect something together in one place in case it helps anyone else. Here are two commands that appeared to do the deed for me:
sudo apt-get install ubuntu-restricted-extras totem-xine libxine1-ffmpeg libdvdread4
The libdvdread piece is what sorts out encrypted disks and needs a two stage installation, hence the second command. For non-encrypted disks, ubuntu-restricted-extras might be all that’s needed if my experience is typical. Taking things further, Wim Wenders’ Buena Vista Social Club (Region 2 disk) worked when I tried it too. In the interests of completeness, I tried the command combination on a virgin 9.04 installation with restricted and multiverse repositories enabled. Along the way, I spied a whole raft of helper components being set in place and things worked like I would have expected with the aforementioned DVD afterwards. As with anything related to computing, your mileage may vary and, for more information, you can take a look here.