I already started receiving bits and pieces for the upcoming Nikon announcements in August/September - new products will be introduced in two waves that are rumored to be few days/weeks apart. The first wave will most likely be for the new Coolpix/Nikon 1 products (Nikon 1 cameras appear to be on a 1 year refresh cycle). The more interesting second wave will be DSLR related. Three possible products were mentioned to me: a D300s sucessor, a new 300mm f/4 lens and a new DX zoom lens (probably to be sold as a kit with the D300s successor). Please note that even though this is the first time I am receiving any info on a potentials D400 camera, everything is still preliminary at that point and I would rate this at 60% probability for now. I am expecting more detailed info to start flowing in the coming weeks.
A new Nikon D4 based camera is expected later in the year or maybe even in early 2014.