Nikon is prohibited by many security regulators from disclosing inside information that could affect the stock price.There are also Japanese commercial laws that require commercial businesses to perform as they announce product availability. Nikon is very quite about there new product offerings until they have a firm ship date. Nikon has announced, after they announced the shipment date of the D4 that they may have to delay shipments since they have found an issue with the firmware for the camera. Nikon has even shipped cameras before they could provide software for processing the NEF files taken by the camera to meet the expected ship date.
Until Nikon formally announces the product, specs, price and shipping date, I would consider anything presented as pure rumor.
Yes I expect a DX format to replace the D300 series, and would expect the name to be "D400" but that also could change.