Upload & Sell: Off
Personally, I am not ready to abandon Nikon over the lack of a D400. I still have my fully pro DX camera, the D2x. However, Nikon needs to show the DX folks some pro/prosumer, metal-bodied love soon. A high-end mirrorless pro/prosumer DX camera body from Nikon would be even more awesome than a DSLR in my opinion.
My only concern with a mirrorless pro-level DX body (assuming no flange change) is the electronic viewfinder. It would have to honestly be high resolution and lightning fast to satisfy me when shooting sports. Now, if they could solve that problem, I'd be all for a mirrorless DX body. Less moving parts, less dust issues, higher frame rates, and it would not even have to black out the EV when shooting so keeping subject in frame would be easier.
That said, I'm not about to leave Nikon over not having a D400 (mirrorless or DSLR) but I will be highly aggravated and it likely limit my future investment in Nikon gear. So, for instance, I'm looking at an UWA DX lens ... D400 comes out I'll pick up either a 10-24 or a 12-24 from Nikon. D400 does not come out ... there are other cheaper 3rd party options. I vote with my $$$.