Hi,
I made a quick comparison of the Pentax 645D vs. the Nikon D3X, based on images from "Imaging Resource". The question I asked was essentially if we can see a significant difference in A2-size prints. The method used was the following:
Downloaded two similar test shots from Imaging Review, both in RAW
Made three different "developments". To begin with I white balanced all the pictures on the color checker card second gray field from left. After Develop->Setting->MatchTotalExposures was done. Identical crops were than established.
Both images were sharpened with Lightrooms Narrow Edge Sharpen. The Nikon image was additionally sharpened using the deconvolution option ("Detail slider" fully to the right" and a bit extreme setting).
The images were exported for printing on glossy paper at 480 PPI in A2 format. Normal sharpening for output was used.
The images were than checked out at actual pixel in Photoshop, see the enclosed screen dumps. I looked at the central part and at the scale at the right. Images at top are Nikon and bottom Pentax 645. The deconvoluted image is on the right. This is extreme pixel peeping! When looking at the images at this absurd magnification the Nikon image has a lot of jaggies and artifacts on the scale.
I also made A4 prints from approximately half of the image. This is quite good approximation of an A2 print. The differences are actually quite small in print. I guess they would be hard to tell apart. The scales on the Pentax image are darker.
To sum it up, I have little doubt that the Pentax images are better but I'm also quite confident that the differences are very hard to see in an A2 size print.
Addition: 2010-12-14
Now I also made 1/4 crops, corresponding to A0 prints. In this size the Pentax image is smoother but the deconvoluted Nikon image still holds up. Disturbing moiré is now visible on the Samuel Smith label.
Best regards
Erik