I have now waffled twice. I promised myself to wait for the D400 to replace my D300 (I shoot mostly birds and other wildlife). Then, I ordered a D800. Then I cancelled it.
Why did I order the D800?
Reason #1: better low light performance with slightly better pixel density, plus all those extra pixels around the edge to catch birds in flight.
Reason #2: who knows if a dx D400 will ever materialize, and it might be a really long time to actually find one even if it does.
Why did I cancel the D800?
Reason #1: I love shooting with a 300mm f/2.8 (VR1) + TC 1.4x, 1.7x and 2.0x (III) because it's the right compromise (for me) between reach and maneuverability (I basically bring it with me whenever I hike, and sometimes when I'm in the kayak or on the mountain bike). However, even with the similar pixel density, I'm thinking the 500mm f/4 might be a better compromise if I switch to the D800. I don't think it's entirely logical, either, given the similar pixel densities. Whatever - the 300mm f/2.8 vs 500mm f/4 remains the source of my greatest ambivalence, but I'm somehow more settled on my current lens when I think about using the D300 (or dx D400) instead of the D800. Plus, $9k for a new lens seems crazy if I'm this uncertain about the advantages of a different compromise between reach and maneuverability.
Reason #2: my D800 order did not become a D800 quickly enough to prevent me changing my mind.
Reason #3: my new full-suspension mountain bike (which costs the same as the D800!) will more easily get me closer to places with birds and wildlife than either a new camera or new lens.
So, I'm now back in saving mode, and I'm pretty sure the D400 (or whatever) will be announced before I've saved enough pennies for it. I'm nearly certain that the D400 (or whatever) will be announced before I have enough pennies for a D800 plus 500mm f/4. Fingers are crossed for a 24mp dx D400 with the ability to bundle pixels for great low-light shots plus reasonable availability by, say, January...