your Pentax picture, normhead, looks like it is a JPEG generated by the 645D, not by i.e. Photoshop, taken directly from the RAW-file. Huuuge difference. I never shoot JPEG with my 645D cos the compression is plain awful. Nikon does a way better job there, at least my D700 does. But the RAW file of a 645D (with a good lens on it) will beat your D800 RAW by far.
I don't care if the 645D will last or not. It is a great camera NOW, I got great lenses (67 and 645 glass) NOW, and 40MP is definetely enough for me, NOW and probably for the next three, four years. The more pixel you get, the more money you will have to invest into a faster computer with insane amounts of RAM and harddrive capacity. A "normal" photographer can do these kinda hardware upgrades only every 5 to 7 years.
Btw, just looked at the sample images on the Nikon site again. I don't get the fuss about the D800. This sample, for instance (
http://chsvimg.nikon.com/lineup/dslr/d800/img/sample01/img_02_l.jpg) shows clearly that the 645D can do a lot better job here as far as sharpness and details are concerned. The library picture looks plain awful to me and doesn't even come close to what a 645D could do here.