actually i think we cant really compare pixel by pixel.
sensor size will also make a lot of difference.
although the d40 is only 6 mp, its sensor size is like almost 10 times much larger than conventional pns cameras.
therefore, it can capture a better dynamic range pictures with less highlight clipping than pnses.
cos if the sensor size is small and have like 12 mps, its photocells get much smaller and therefore less sensitive to light which then contributes to noise.
try comparing a 800 iso pic from a dslr and a pns(except fuji f31) then you'll know the difference.
the largest sensor for a pns ever is the sigma dp1(full frame) which is not released yet. it has a 12mp sensor but only have a 4mp effective pixels.
if you really wanna print a good quality poster sized photo, you must go for full frame and above.
one of the best to do that is the hasselblad cf-39ms digital back which sports a 39mp medium format sensor. the details of its pics are just simply stunning.

captain obvious had just save the day!