Total speculation on my part, but I consider it a given the D400 will match D800/7000/V1 battery, 400/800 will match on grip, 400 will have improved AF, EXPEED3, and subsystem improvements of D800.
Will make for a compelling case for many to get a matched D800/D400 pairing just like the D700/D300 were so well matched as shooting companions.
Am still not sure about a D400 sensor, mp, or fps but these three features go hand in hand. D400 will almost surely be at least 5fps, probably at least 6fps though. and then more with the grip (good motivation to get the MB-D12 grip).
I also consider D400 will have to be DX camera, but I know many hope it will be FX. A DX sensor with Nikon D3S/D4 technology low light sensels would be ideal (but I doubt it somehow).