This is Alienware's Steam Machine in every way except technically: It won't ship with a Steam controller and it won't ship with SteamOS. These are the two things that make a Steam Machine, according to Valve.
"It's absolutely Steam-ready," Alienware Global Marketing Director Bryan de Zayas told Joystiq on the E3 show floor. "It's not that we're bypassing Steam. It's critical. They have the majority of the digital downloads."