For full frame and getting to 12, the Sigma 12-24 is about the only option. The problem is that it's optimized for crop bodies and is really soft around the edges.
It also looks like the Nikon 14-24G and an adapter is a pretty solid combination but I don't think that I want to go that route. I think that I've talked myself into the 15 fisheye. If I need to, I can remove the "fish" with DxO after the capture.