You know Imkeww, I'm not exactly sure. I saw the same thing, that's where Midwest Express and US Airways (at least) have their parking spots. I'm surprised it hasn't already been demolished. My best bet is that when Concourse D has been re-modeled and opened, those airlines will take that place, but it's also possible that they will try and accomodate them to other gates and demolish Terminal A so as to let Terminal A operate at full 12-gate capacity.
AerLingus: Oh yea, forgot Alaska Air, and Air Canada/Canadian
those gates are closest to the terminal (flights from Seattle usually park at Gate 20, the closest gate, flights from Vancouver usually park at Gate 22, others I have no idea
Head in the clouds... yet feet planted firmly on the ground.