I've used my D40x (more MP, but D40 has enough MP as it is) to shoot photos for a certain magazine.. I mostly used a non-professional 18-200mm VR lens too. later on, I added an SB800 to the combi.
Even without the SB800, i could take indoor shots which were suitable for publication. I won't say either the mag or my shots were of a high standard, but u get the picture.
istockphoto has accepted two photos which i shot with my d40x (one of them taken at iso800. d40x is noisier than d40, btw). the camera can easily produce "stock quality" images, but it was only limited by my ability. i wun even say it was limited by my then-meager collection of lenses.
it's always about the photog. but some pple feel better with a big "pro" camera, INCLUDING MYSELF. but i always remember that "feeling better" never translates to taking better pictures. but i upgraded anyway cos i wanted more functions for other kinds of photography.