You need the libdvdcss2 package to play encrypted DVD's. Search the forum to see where to get it from. It's not in the regular debian mirrors, so you'll need to alter your /etc/sources.list. Either that, or just grab the .deb, and install it with "dpkg -i libdvdcss2*.deb"
You can find libdvdcss2 by searching the unofficial apt sources at www.apt-get.org