Down in Shreveport, Louisiana, Hollywood entertainment professionals are shooting a film called Battle: Los Angeles. The film is due in early 2011, and will apparently feature the above billboard for Resistance 3 in the background of at least one scene. As you can see from the billboard, Resistance 3 is for the PlayStation 3 and is from Insomniac Games.
Does this mean a sequel to Resistance 2 is definite? No. It doesn't. For all I know, Battle: Los Angeles is set in Opposite World, where all billboards lie. Also: Sneaky film-makers have been known to use movie-magic technology to change the content of billboards in their shots.
But on the other hand... the Resistance games are very popular, and video game developers and publishers like making money; plus, the timeframe seems about right, so, if I had to make a bet, I'd bet there will be a Resistance 3, and the movie billboard is a little bit of product placement.
Although there are no pictures of billboards to go with them, I'll go ahead and predict Halo IV, Grand Theft Auto 5, Madden 11, and a new Tetris game right now.