Tetris: mere ruleset or copyrighted expression?

What makes Tetris Tetris? The mobile app explosion threw gasoline on the game's already-numbing history of copyright battles. At Ars Technica, Kyle Orland takes a look at the Tetris Company's endless efforts to kill them. These efforts hinge on a seemingly straightforward question: is Tetris simple enough to be defined by a set of rules (to which copyright cannot apply) or does it qualify as protected expression?