That seems to indicate that Ramsey exists in the HBO Westeros universe. I think the most likely scenario is that Ramsay is a glorified extra in season 2 (in full armor), and then is cast for season 3.
I can definitely see why they did it this way, but I'm worried about how they are going to pull off the millers children event. I just don't believe it in Theon's character to actually do the deed. Turn aside while Reek does it? Yes. But to actually do it himself I don't think so.
Hmmm...I just thought of something. Black Lorren was cast and that the guy who talked about taking Theon's ship was almost assuredly him. He could be the person to suggest/carry-out the act. It is a key characterization event for Ramsay, and I hate that it is going to be shifted to someone else, but I think this might work plot-wise.