I haven't come across anything official, but it's very likely that the information you have now is correct. I should imagine they'll release a boxed set for the theatrical versions and a separate boxed set for the extended.
This is only speculation, though -- I don't know if it's a fact.
Viggo Vixen, Supplanter of Arwen & Anti-Hugging Harpy Posts: 265 Send Message
RE: DVD Boxed Set on: March 23, 2004 11:21
According to this article from DVDAnswers
The theatrical trilogy boxed set is said to be realeased on May 25th, along with the release of the RotK theatrical DVD. It is said that this will not include any extras, but will simly be the same as if one bought all three theatrical dvd's seperately (just in a nice box, and probablly a bit cheaper.)
Edit: You can pre-order pre-order the Trilogy Boxed Set of the Theatrical Editions (this means no new special features) at NewLineShop.com. It ships on May 25th, same day as RotK Theatrical DVD release.