I may have confused things slightly. I meant to say that one should create a 1,200 pixel-wide jpg (the kind of image we might see posted on Nikonians) and in a completely separate processing step, create a 10" wide print (or make it 12" if you like) at 300 or 360 dpi. I did not intend to suggest printing the 1,200 pixel image at 10 inches wide.
It seems to me that when downsampling, I'm actually going to lose more image information going from the D800 native resolution to 1200 pixels than I would going from D300 native resolution. The same holds true for a 10-inch print. I'd be throwing away more pixels from the D800 than I would with the D300. Wouldn't that make irrelevant any extra resolution available in the D800 original image?
Now, in a 12-inch wide print, the D300 would have to be upsampled slightly to print at 360 dpi (or printed at 357 dpi) whereas the D800 wouldn't. Seems like that might be where one would begin to see a difference (although I think it would be pretty minimal at that size). I would assume there would be more of a difference as print sizes got larger.