At this point in time, it's unclear who Arnett will be playing, although THR has said to rule out the possibility of Casey Jones, other half to Megan Fox's April O'Neil. Both Shredder and Splinter remain uncast, and with Arnett having extensive voice work experience (see the likes of Ice Age: The Meltdown, Ratatouille, Monsters vs. Aliens, Despicable Me), the TMNT's rodent master could be a good bet.
Filming on Ninja Turtles is expected to begin later this month, with Arnett and Megan Fox joined in the cast by Alan Ritchson (Smallville, The Hunger Games: Catching Fire) as Raphael, Pete Ploszek (Parks and Recreation, Shameless) as Leonardo, Jeremy Howard (Galaxy Quest, How the Grinch Stole Christmas) as Donatello and Noel Fisher (Hatfields & McCoys, Battle Los Angeles) as Michaelangelo. The film is scheduled for release on June 6th, 2014.