Square-Enix detailed the contents of "Professional Edition", the European collector's edition on their blog today.
The Hitman: Absolution set is a bit more down to earth than say, the Resident Evil 6 "Premium Edition", which was announced earlier today. On top of the game, "Professional Edition" adds the DLC "Agency Gun Pack", which gives you three extra guns: The Jagd P22G pistol, the UMP SMG and the SPS 12 semi-automatic shotgun. It also comes with a concept art book, a making-of documentary on DVD, and a nice clamshell box.
According to Joystiq, Square-Enix has already said that an American collector's edition is on the way, but it may differ from what's in this set.