Don't worry, it's coming...
Apparently, QuickTime 5 will be getting support for Sorenson 3, MPEG-2 and MPEG-4 codecs. Apple just couldn't finish development before they wanted to release the new version.
Now, this sounds like you will be able to "IMPORT" or "EXPORT" a QuickTime file using those codecs, but unless you have the correct video hardware (with MPEG decoding) I doubt you'll be able to actually watch and listen to a commercial DVD with the QuickTime Player. You'd also need the decryption of the movie data that the DVD Player provides and the ability to handle menus and multiple files.
So, I guess we'll just wait and see.