I would second VLC which will play DVDs from any region. It is more CPU intensive since VLC has to do all the decoding in CPU but a new PB should be able to handle it fine. I have a couple Region 2 DVDs of things that are not available in the U.S. and VLC handles them fine.

Mplayer is another option. I have not tried it on the Mac but it works well on Linux. Downloading it now just to compare with VLC.