My experience is that clients are going to be much more concerned about art than the laws of physics
I am confident that I could shoot a wedding with D40s (at least 2), and the clients would be just as happy as when I shoot with my 5D.
I agree that they are not concerned about physics, however , physics will define the quality of your pictures! Can that D40 take clean ISO 1600 images? A D300 is good to 3200, a D700/D3 are EXCELLENT at ISO 6400 and produce less noise at 12,800 than the D40 does at ISO 1600.

... I challenge any D40 to come into any local church in the area that require ISO 3200 just to get a shutter speed of 1/60th and try to get the same quality shots that a D700 could get at ISO 3200, 6400 or higher. I challenge any kit lens to show me equal distortion, CA and pin cushioning numbers as a Nikkor 70-200 or Nikkor 85mm F/1.4 lens. We're not even in the same country, much less the same ball park... lol.
I challenge any D40 in camera processing to match the quality of output of the in camera processor and the full frame sensor of the D700 (same as the D3). I challenge any kit lens to be as clear and distortion free as a quality L or "gold ring" lens.
If we are of about the same technical and creative level and I carry in my D700 and 4 lens collection (14-24, 24-70, 70-200 and 85mm) into the same wedding as you do with your D40 and kit lens, and we go side by side through the whole process... and then show both results to the same client, I sincerely wonder which set of pictures would make the client happy and which would make them frown?
It really is a lot more going on inside the camera than just high ISO and low noise, though that is a big part of it.
Not even discussing the D40, if you join the D700 flickr group or read the D700 Nikonians forums, you will hear how people are even loathe to pick up their D200s and D300s after playing with the D700s, and we are not even talking about how much further behind the D40 is.
A D40 at a wedding against proper equipment is like trying to race a stock Yugo in a Formula F1 race. Not only are you not in the race, you are not even able to keep up to the pace car!

Can the Yugo run on the track? Sure, but it's not competitive and sure as heck NOTHING it can do to win that race. That is a D40 in a wedding environment against even a D300, and the D700/D3 are above that level by a far margin... those are the real competitors in the wedding field.
If there were no differences between a 5D (or any professional level camera), and a D40... why would any professional ever need waste more than the price of a D40 and kit lens? Because there are differences and major ones at that.
