I received this tip two weeks ago and was waiting and hoping to receive some more information before posting, but unfortunately I did not get any - hopefully this post will spark a new wave of rumors:
Nikon D3 successor will appear on Autumn. It will still contain 12 mp but with dust removal system.
The tipster mentioned also the possibility of a 24 mm f/1.4 lens. I will not comment on the source, but I have a reason to believe that it is reliable.
I still have the feeling that the D300 and the D3 will be refreshed at the same time (they were released at the same time) with some minor updates (see potential Nikon D300s specs).
Nikon Rumors rating system: 60%