>Building essentially a D4, but with faster data throughput, >and more compact since the D400 probably would not have the >extra space in the integrated grip, the D400 is not going to >be cheap to make.
I never subscribed to the cheap-cheap price for a D400 theory. However if one looks at the announce price of pro-dx cameras in the past, I expect to pay between $2000 and $2400 for such a camera if it ever is released.
Can they fit the expected specs within that price range? Well, compare the D200 to the D2X, and the D3 to the D300. I think they can pull it.