To my way of thinking, Steam would most certainly improve Linux's attractiveness to developers, but to think it alone would fix everything is probably misguided and overly-hopeful. That will require a convergence of several factors. The porting of Source and the Humble Bundle numbers might well do more for Linux gaming than Steam and Desura combined. Getting a native client for the current Unreal Engine would kick ass, too. But as they say, one step at a time.
No announcement yet.
It's Official: Valve Releasing Steam, Source Engine For Linux!