I've gotten DVD playback to work under Debian Testing on my Powerbook. It was relatively easy, although I couldn't do it just with the package manager.
I had VLC installed, so I first tried to use it. I told it to open /dev/dvd, but it refused, saying it was scrambled, which I expected. I knew I would need a DVD descrambling library, and so I searched around and found that what I wanted was "libdvdcss2". Debian won't distribute it since it's illegal under the DMCA, but it's pretty easy to find on the Internet. I found this mirror from googling "libdvdcss2 powerpc". I installed libdvdcss2_1.2.3-2_powerpc.deb without a hitch, so I ran VLC again and pointed it at the disk drive, and it worked!
This isn't the end of the story -- the sound and video worked, but every few seconds the video would hiccup. And believe me, when you're watching a video, these hiccups are really distracting. It sounded like my CPU might not have been fast enough to play fullscreen DVDs with that player, or something (it works great in OSX though). I figured I could try another player called Ogle on the recommendation of a friend. There was a version of Ogle optimized for the AltiVec vector processor in my machine, in the package ogle-altivec in Debian, and I was hoping that at least the AltiVec stuff would make it go faster. I installed it and it worked perfectly. Yay!
I wanted to relate this story because it has a few morals: First, it's actually reasonable nowadays with GNU/Linux desktops to fix things that don't work "right off the bat" with a minimum of pain -- even when the package you need isn't part of the distribution, it's easy to go find a version you need on the net and install it manually (and if you don't know what package you need, reading pages online usually gives a clue). Secondly, it's a lot of convenience to be on a platform where there are plenty of free alternatives for different software tasks -- when one program to do a task doesn't work right, there are very often other programs you can install that do it better. Lastly, the DMCA is a very bad law, and I encourage you to break it -- I broke it by installing libdvdcss, but the law really shouldn't exist and is probably illegal under the First Amendment.