The one pilot said "I guarantee you that a wild animal could not open that buckle", so that's pretty much right out there saying that Fossett would have been the only thing capable of manipulating it. Whether he never buckled himself in, or it unlatched it before crashing, will never be known. However, from the pictures shown in the program, the buckle was very close to some major parts of the aircraft. Leading anyone with a basic knowledge of physics to realize that the part had to have been dropped there. Otherwise, it would have been propelled farther away from the crash site.
I was thinking when they were showing the video of buckle, I would have done something more substantial with it that would just scream "I survived". Like, putting it on a tree limb, on what is left of a wing, etc, so that people could see that it was placed there.