This is my fourth update here.
The first update was to say that I am having the same issue: Bad quality (235kbps on test video) on my ATV 3, Comcast at 15-10Mbps download speed, perfectly fine Netflix performance on iPad, iMac and Macbook Air.
The second update was to say that I ordered a Roku on cyber monday, despite not wanting to switch.
The third update was when I chatted with Netflix's customer support, and they suggested I cancel the service.
In this, my fourth and most riveting update, I'm going to tell you all that the Roku 2 works awesome on the same network. Nothing has changed except that I swapped out the ATV for the Roku. It is 7:00pm here in Minneapolis (why are there so many Minneapolis people having problems?), and I a getting a steady 3000kbps on the test video. (Netflix cancelled my billing, but I still have the service until the end of the billing cycle).
Definitely an Apple TV <--> Netflix issue, from what I can tell.