After the recent and likely fictional Nikon Roadmap Leak (see Nikon Roadmap in our forums for the full translation), Thom Hogan has put together his speculated "Possible Nikon Roadmap".
Note Thom does have good sources, and a pretty good track record when it comes to predicting Nikon products, so his version of the roadmap is probably best described as "educated speculation".
Thom speculates mid 2011 for the Nikon D400 release. He expects:
New technologies spawned by the D4 design group passed to DX
His full roadmap with a lot more background info and explanation can be found by clicking the source link below.
Source: Thom Hogan on 7/14/2009.