Downey Jr. is producing the movie with his wife Susan Downey (Gosh, it's weird that RDJ's last name isn't Downey Jr.) under their company Team Downey, with the help of Warner Bros.
Now, while he's only announced as a producer currently, there is also an inkling that RDJ may play Phil Kitzer, the titular Phil in the movie. The movie is based on a book, titled Chasing Phil: The World’s Greatest Con Man, Two Undercover FBI Agents, And Their Amazing Around The World Adventure by David Howard, itself based on the true story of Jim Wedick and Jack Brennon. The rights for the book, as well as the two FBI agents lives, have already been purchased by Warner Bros.
The plot, as you may have guessed from the book title, follows Wedick and Brennon as they go undercover in Phil Kitzer's Fraternity of conmen, which is running multi-million dollar scams. However, how closely the plot follows the book, or even reality remains to be seen.
Don't expect to see Chasing Phil any time soon, given how early it is in the process, however we'll keep you up to date on any news relating to the film.