Yes, there is a Sly Cooper movie in development. It a very real thing and we now how official proof in the form of the a fancy movie still and also the movie's debut trailer. It's currently being made by Blockade Entertainment, who are working on the Heavenly Sword and Ratchet & Clank CG movies. Expect the Sly Cooper Movie to arrive in theaters in 2016.
Unfortunately the trailer tends toward more typical children's movie trailers, which is to say it's likely not an excellent way to gage the content or quality of the actual film. It does, at least, introduce us to our protagonists. We've got Bentley the turtle, "The Brains", Murray the hippo, the muscles, and of course Sly Cooper the raccoon, who's everything else the gang needs at any given time. Also, they drive a van.
"In the film, Sly learns of his birth family’s secret legacy; that he comes from a long line of talented and international thieves. Endowed with this knowledge, Sly and his friends are catapulted into a global adventure as they race to reassemble pieces of an ancient book holding The Cooper Clan’s family secrets before it can fall into the hands of Clockwerk – an evil Russian metallic owl bent on ending the Cooper family line."
Check out the Sly Cooper Movie in 2016, when I assume it will come to theaters in some sort of limited release? I'm not sure if this is going to be any sort of big time film. I suppose time will tell!