Any copy protected DVD is actually encrypted using a scheme called CSS
(Content Scramble System), this requires a decoder to play licensed DVD
content.

The following URL's are just for reference, if you want to know more
about CSS and what it did/was supposed to do (I find it kind of
interesting).
en.wikipedia.org/wiki/Content_Scramble_System
http://en.wikipedia.org/wiki/DeCSS

Do you have the CSS decoder library installed?

If not, search for "libdvdcss2", to get it you may have to add the
Medibuntu repositories to your system...

http://www.medibuntu.org/

However, it should be noted that legally speaking libdvdcss2 violates
the DMCA, as the DMCA (here in America at least) makes anything that
hacks or breaks copy protection schemes no matter how old they are a
federal offense. That said, there are no actual Linux DVD decoders out
there to my knowledge, making libdvdcss2 the only option for Linux users
wishing to watch copy protected (any) DVD's.