I would have assumed the D3 would be the best landscape camera for now since it has a full frame sensor it would allow for true 12.1 mp shots with the widest fov and lowest noise. I also would have thought the D2x would be a good camera to keep for sports BECAUSE of its 1.5X crop factor and the 2X crop in HSC mode.
I will likely get the D3x when it comes out since it will no doubt be in the 24+ mp range and be optimized for landscape/portrait work. At that point the D3(s) would be able to back each other up to some extent if one failed but clearly they couldn't replace each other.
However, I should keep a APS-C camera to take advantage of the high pixel density in the small sensor size. This gives me the 1.5X advantage with full resolution on the image. I have a 200-400 that becomes a 300-600 or 400-800 f4 in HSC mode. I have a 200 f2 that becomes a 300 f2. Not to mention I have a 12-24 and 18-200VR that are great lenses. So there is no doubt I want to keep an APS-C camera. The question is in the role and line up described above, would the D2x or D300 be a better choice.
Good judgement comes from experience, experience comes from bad judgement.~ Mark Twain