Why hasn't Nikon released a f/2.8 DX zoom?

No, I think without a doubt any D400 would have a motor. The D400 would be a progression from the D100, D200, and D300 which were the top non-pro DX cameras and all had focus motors.

You see, that's the problem. The D300 actually has a better build quality than the D600 has. The D600 is an FX D7000 which is down the food chain from the D300 or hypothetical D400. That's why I don't think Nikon will make one. Why create a better build DX camera from your new bottom FX camera. Too much overlap.

--
Cheers, Craig
Follow me on Twitter @craighardingsr : Equipment in Profile