If you're arguing that Pruneyard Shopping Center authorizes you to sell a photo of Cinderella Castle, then you are stretching it much too far. In that case, the Court held that individuals could peacefully exercise their right to free speech in a mall. More specifically, it held that the affirmative right to free speech contained in the California Constitution (as distinguished from the negative restriction on the government found in the First Amendment to the U.S. Constitution) did not infringe the shopping center owner's property rights under the 5th and 14th Amendments. So I guess maybe you could go yell about healthcare reform in front of the castle at
Disneyland (not sure what the Florida Constitution has to say on the subject) . . . or maybe not, because this still ignores the distinction that you've had to purchase a ticket to enter the theme park, and that ticket includes a long list of terms and conditions. Like rtphokie, I was not able to locate those quickly, and unfortunately I don't have the time right now. My guess is that by entering the park using that ticket, you're agreeing to some restrictions on what you can do with the pictures you take there, but I really don't know.
On the trademark issue, there was a decision (6th Circuit, I believe) where the court refused to recognize the Rock and Roll Hall of Fame as a trademark. Obviously, that doesn't mean that a different court wouldn't reach a different conclusion about a Disney icon. I did a quick search on a stock photo site for a "Disney castle" and came up with a lot of pictures of the Neuschwanstein castle in Germany, but none of Disney's. Evidently, the threats are effective, even if the law is unclear.
Here's an interesting article on the need for property releases, acknowledging that the law in this area is quite murky:
http://www.asmp.org/tutorials/using-property-releases.html I think the conversion theory is interesting--analogous to an individual's right of publicity--basically that by profiting from an image of someone else's property, you're converting that property to your own use.