Michael Bay's Ninja Turtles has reportedly cast Will Arnett in a role that's not being revealed, but which is being categorized as a lead in the film, according to The Hollywood Reporter.
Lead roles yet to be cast are Casey Jones, Master Splinter and Shredder, although the article notes that Arnett will not be playing Casey Jones. Of course, Michael Bay added extra human point-of-view characters to the Transformers films, so there's really nothing to say that Arnett has to be one of the remaining two.
THR refers to the film as "Teenage Mutant Ninja Turtles," which is interesting; when it was first reported that the turtles would be altered to be members of an alien race rather than earthbound mutants, Michael Bay confirmed those rumors, and that the film would be titled simply Ninja Turtles. Recent comments by Bay and others associated with the Jonathan Liebesman-directed live action/CGI-hybrid film, though, have consistently referred to it as "TMNT."