not necessarily public beta v6 but first step in that direction.
However UPDATE_en.txt the says:
EAGLE Version 5 Update Information
==================================
This file contains information for users of previous EAGLE versions.
Please read this file entirely if you are updating from an EAGLE version
prior to 5.91.1!
WARNING: The data structure in this version is completely different from
that in older versions! Once you edit a file with version 6.x you
will no longer be able to edit it with versions prior to 6.0!
PLEASE MAKE BACKUP COPIES OF YOUR CURRENT BOARD-, SCHEMATIC- AND
LIBRARY-FILES BEFORE EDITING THEM WITH VERSION 6.0!
and indeed the file format is xml.