Yes, but Google could still collect ad revenue on non-tracked users, they would just be restricted to the same kind of ads they use to target non-logged-in browsers. Obviously the ads within Gmail would be worthless on an untracked user - nix those. But the ads on the Google search pages? On sites that host AdSense (or whatever they call it now)? They would still have tremendous value.

I do wonder if this wouldn't break Google Now though.