It should first be stated that Hill's hypothesis is based on a number of things – firstly, the photo that Brad Bird posted to Twitter (see above), showing the box (marked "1952") that initially piqued the interest of Lindelof. The story goes that during a story meeting with Disney executive Sean Bailey, Bailey drug out the mysterious box, the contents of which belonged to WED Enterprises (WED stands for Walter Elias Disney, and WED Enterprises was the prototypical company that we now know as Imagineering). The contents of this box (originally labeled for cornball 1965 live action Disney movie "That Darn Cat" but since relabeled "1952") were what would form the basis for Lindelof and Jensen's original script.
This box was heavily analyzed early last week on the website for official Disney fan club D23, by Walt Disney Archives director Becky Cline. Among other things, she contended that the box contained photos of Walt Disney with visitors to the studio, most notably Major Woodlief of the U.S. Army Reserve General Fund, a blue paperback book (clue!), a copy of "Amazing Stories" magazine from August 1928 (the same issue that famously birthed Buck Rogers), a box of slides from Technicolor, and a 45 vinyl.
Based on this information (the blue book obviously a reference to the infamous Project Blue Book, an Air Force project meant to assess the validity of reports of flying saucers), coupled with stories that legendary Imagineer Ward Kimball (a former mentor of Bird's) used to tell, and Hill has surmised that "Tomorrowland" is actually about Walt Disney's involvement with an Army project to let the world know that UFOs really exist.
The story told by Kimball and others over the years goes like this: Supposedly after the "Man in Space" episode of the Tomorrowland-inspired "Disneyland" TV series (see below -- the show racked up over 42 million viewers and, according to some members of the Eisenhower administration, played a key part in America's decision to enter the space race), various government officials approached Walt with the idea to do a show about UFOs. It was their hope that, through the fanciful eyes of Walt Disney, he could release this information in a way that wouldn't cause mass hysteria or social upheaval (the documentary would touch on the fact that aliens have been visiting Earth periodically throughout history.) Supposedly the episode, according to Hill and his various sources, was canceled when the government types refused to release actually footage of UFOs that they had amassed, coupled with their claim that, while they wanted Walt to produce the documentary, they wouldn't do it unless they themselves fully understood what the UFOs really were and how they worked.
Hill also points out that Damon Lindelof's tweet about the project (saying that it's "NOT about… ALIENS") is a purposeful misdirect, since "Tomorrowland" won't be about aliens… It will be about UFOs. Either way, we'll have plenty of time to pour over details and speculate wildly as "Tomorrowland" won't hit theaters until December 19th, 2014.