Buy Blur on Steam before it comes out tomorrow and you get Geometry Wars for free. Bargain.
It's here. While this is being reported all over the shop, however, we can't actually find a button that allows us to buy it. Maybe you'll have better luck.
Blur, Bizarre Creations' shooty racer, releases for 360 and PS3 this Friday, if you're looking for an alternative. Don't think you get the studio's ancient space shooter with it, but what the hey.