According to TVGuide, he'll appear in Episode 2, "Secret's Safe With Me." His character, Marco Massetti, is described as a "career criminal who runs afoul of Castle and Beckett after they discover their murder victim may have been involved with one of Massetti's jobs."
Sounds like it's back to crime solving as usual for C&B! Except for the occasional makeout session in the interrogation room (we're assuming).
Castle returns on Sept. 24, 2012 at 10 p.m. ET on ABC.