Most DVDs you will come across are CSS encrypted. When you rely on a open source operating system that aims to be free, like Linux, this can prove to be a problem for you. Just run the following in a terminal to allow Ubuntu to play commercial DVDs.
sudo apt-get install libdvdread4
You may need to reboot before these changes will take effect. This should work for both 32-bit and 64-bit installations. I tested this on Ubuntu versions 9.04, 9.10, 10.04, and 10.10.
In some countries this is illegal. Take a look at your local laws to make sure you will not bring the fuzz down on yourself.