It will be a box set of all 6 movies, including updated scenes and effects of all 6 & plus the O-OT as a seamless branching version. It will also have the deleted scenes from the OT we didn't get in the 2004 DVD release, and new featurettes on the saga as a whole. This way Lucas covers all markets: He gets the fans who want a box set of all 6 movies (prequel gushers), the fans that were waiting for all 6 to be released (small group, but there are some out there that are still waiting), and all of the disgusted OT fans who are clamoring for the O-OT on DVD. This way, it will make everyone happy in one big box set, and it will cover all bases of SW fans.
The theory I think on why they will release the O-OT on DVD with the boxset is, they know many fans who are disgusted with the PT, won't be interested in a new boxset, but putting the O-OT with those updated movies, how can we resist?
It is just a theory, but as much as I bury Lucas, and feel he has lost it sometimes, he is a damn good marketer, and he knows when SW can make money, as Yoda would say, "Making money, that he has never lost the ability to do!"