The new Nikon 85mm 1.8 ($500) is much cheaper than the Sigma 1.4 ($970). That's the main reason.
Resale value on the Nikon will be better later on, that's another.

I have a 85mm 1.8 AF-D now. I like it, but it doens't autofocus on the D5100, which is the camera that I use the most for portraits and low-light. This new 85 will work on all of them, so I am only spending a little more to get it, if I sell my AF-D.

If I don't like the new Nikon 1.8, then I will get the Sigma 1.4 later (haven't ever shot the Sigma, but that's a lot of money for a single lens). I won't get the Nikon 1.4, as it is just too much money ($1700).

Once it warms up, we should try to get the Nikon shooters together, and bring out all of the lens. I have quite a few that people could try and see what they like to shoot, and I'm sure that others do also. I bought my 85 1.8D without trying it first, and didn't know how much a pain it would be to manual focus it. It's great on my D70 and N70 though. Now I am glad that I didn't get the manual focus Rokinon 85mm 1.4. The only MF lens that I like is my 500mm. Everythign else is too hard to get sharp when you are in a hurry to get the shot.
Do you think that a IA/Nikon get together would be a good idea? Maybe at Stone Mountain, or downtown Atlanta (somewhere with things to shoot, and places to park close together)?