I would expect the D400 to be made in Thailand just like the D200 and D300. This dealer sounds like he's making things up as he goes along. However, given the troubles in Thailand maybe Nikon was forced to shift manufacturing to Japan.....so maybe there is some truth to this.
If they do have to manufacture the D400 in Japan I would expect one of two consequences. The first is that the price is going to be higher than expected. I would expect a D400 to be just shy of $2k. If made in Japan it may go higher. Which would stink...and start to encroach into D800 territory. OR Nikon makes them in Japan and eats the higher production costs.
Hope it is option number two. I really don't want to have to agonize over D400 vs D800 if they are close in price.
For what I do (right now) I prefer the DX format.....but the D800 can be shot in DX mode.....