My workflow, which makes great 24x36 inch landscapes from D3x files printing to the Canon iPF 6100, is as follows (it's clearly related to a lot of what other folks are doing):
Convert in Capture One - sharpening on (C1's default is pretty darned good for capture sharpening from the D3x). I try to get basic exposure and white balance (including any curves adjustment I want) right in C1, leaving as little global adjustment as possible for Photoshop (all that I do globally in Photoshop is apply any custom filters I want, such as PhotoKit Color). I don't resize at all in C1, and I output a 16 bit TIFF in ProPhoto RGB.
Adjust anything needed in Photoshop CS4 - filters C1 can't handle, local adjustments that need sophisticated layers, etc...
Resize to output size using Genuine Fractals 6.0. I actually go all the way to print size at 600 dpi (the Canon plugin is 600 dpi native, which I have confirmed multiple times) - there is a small but noticeable improvement by doing this over resizing to 300 dpi and letting the Canon driver handle the final resize to 600 dpi. Beware the 1.67 GB file!
Output sharpening with PhotoKit Sharpener Inkjet 480 glossy fine (I know it should be 600, but that doesn't exist)
Print through the Canon 16-bit plugin - iPF6100 on Hahnemuhle PhotoRag Baryta.
If the D3x has limits, I haven't found them yet - this workflow produces ridiculously nice high-detail landscapes up to 24 inches on the short side (I don't have room for a larger printer :-) ).
-Dan