It's actually anodized. You could make a brushed finish with a bit of DIY elbow grease, though. They look pretty nice that way if you do it right.
EDIT: I've been trying to find the guide I saw for it online. It involves removing all the electrical components from the computer (so you just have the outside shell), brushing it by hand, and re-oxidizing it with a chemical bath. Not for the faint of heart. It's the kind of thing I've been wanting to practice on my older MacBooks if they ever die... but none of them have died yet.