I was surprised to learn that QuickTime 7 (at least in Tiger) has the ability to open and display VOB (Video OBject, see note) files found on DVDs. These can also reside on your hard drive if, for example, you use a DVD authoring system such as DVD Studio Pro. This enables one to preview VOB files within the Finder, a very welcome change, indeed. This new capability extends also to encrypted (including commercial) DVDs.
Note: VOB files are muxed, meaning they contain both video and audio streams. For now, QuickTime simply displays the video information, leaving the sound stream inaudbile both in QuickTime Player and the Finder preview.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050515115816394