I also spoke with one of the employees when I was in the store on Wednesday, and he said they will most likely be arriving late in the afternoon on the 16th. I'm not sure if this is happening with all the Apple Stores or just this particular one (since it is one of the "flagship" New York stores).
Anyway, I'm buying one as soon as I can get in there and grab it. If you live in my area, please leave one for me. I seriously debated not posting about this, just because I wanted to ensure it would be there.
Update: After reading comments from readers that iMacs are beginning to show up in some Apple Stores, I called the Westchester again and discovered that they too have the new iMacs in, but only in the 17" size. I'm gunning for a bigger iMac so I'll hold off. They only had 4 when I called, so you may want to run and get it if you're in the area and looking for a 17".