GameSpot recently discovered the two domains, TheLastOfUsMovie.net and TheLastOfUs-Movie.com, despite Sony taking pains to obfuscate these sites. Instead of registering the domains through Sony or one of its many subsidiaries, the two URLs were registered by a corporation called MarkMonitor which grabs websites like these for clients who would prefer that their name stay out of the same media spotlight we're now shining at them.
That said, it's possible that Sony is acquiring these URLs as a way to protect any future plans it may or may not have to make a Last of Us film. Corporations plan these things long in advance, and there's any number of reasons why a company might want to lock down valuable intellectual property.
On the other hand, The Last of Us was one of the best games of 2013, has sold more than three million units in half a year, and generated more critical hyperbole than most publishers could ever hope for. With all that built-in momentum, Sony just needs to find a talented, young lead to play Ellie. Maybe that girl from Juno is available.