Due to some rather silly and lengthy legalities, Ubuntu Linux does not install in a state that allow you to watch a regular old DVD movie. I have tried without success on multiple occasions in the past to insert a disc to watch - only to have it fail. It never really occurred to me that this was "by design" and I always assumed that it must have been something I was doing wrong. Recently I found that I could play any of the movies that I had created (without encryption) but not a single commercial disc.
After some investigation I learned that you can enable the ability to watch commercial discs rather easily, but it's likely illegal (read the fine print).
Rather than act as if I came up with the solution, I am pasting a link to a pretty good article on the subject matter. While the article was written on earlier versions of Ubuntu, this trick still seems to work on the latest release (Hardy Heron).
How to enable DVD playback