If you grabbed the final release of Ubuntu 10.10 this weekend, you can now fill out its codec compatibility, software updates and DVD playback powers with the Medibuntu repositories, recently updated to support the latest Ubuntu release.
Medibuntu's mission is to provide all the playback and encoding capabilities that most Windows and Mac systems contain, but which Ubuntu's creators decline to include by default in their systems, due to licensing restrictions and a lack of open-source code for those features. If you just want to get AAC playback, copyright DVDs playing, and newer versions of all the audio, video and font files, load Medibuntu into your system.
Medibuntu's wiki page explains the one-command installation. It's free to use, and there's a section on installing just the DVD playback capabilities, if that's what you're after.