It's been publicly stated by Apple employees on the public MRJ mailing list that there are major updates for the Java layer of Mac OS X that just barely missed being included in the public beta.
There is a strong desire by the development team to get these fixes out to the users of the public beta but they describe this as a "difficult" proposition because the Java code is integrated at such a low level.
There are many fixes that people are waiting for: Ethernet settings that aren't in the kernel and don't require a reboot to take effect, robust AirPort support, Java 1.3 virtual machine, faster message passing, better graphics performance, heck even improvements to the software auto-update feature itself. Some of these sound like they'll need to have a CD-ROM update.
Unfortunately, I think that Apple is unlikely to go through the hassle and expense of a CD updater with the "final" release so close. And if these "itchy" bugs aren't squashed by Apple, why would they bother fixing the little things that no one notices. All they'll hear in response is "You fixed problems V, W, X, and Y! Why didn't you fix Z?"
Bug fixing the public beta is a no-win situation for Apple.