While the 40D is a good upgrade from a 30D, its still a prosumer camera, while the 5D is a professional body, although who really cares about that, its just words!
I know if I could choose again, I would be tempted by the 40D, but still end up with the 5D, im solely into landscape photography & I know the 40D + 10-22 will give the same FOV but it just doesnt feel the same to me, the FF sensor is exactly like film SLR's, the 40D may have an improved viewfinder but its still not the same, a 10-22 also doesnt have IS, now some say its not needed on UWA lenses but I dont agree with that, try shooting at f/16 with a CPL & you will soon run into problems.
The other thing I never liked about a 1.6x body was the fact that to get a simple lens like a 50mm, I would have to buy a 30mm (usually a Sigma) which limits my choice.
Lenses like the 24-105L are out of the question as well, on a 40D its just not very wide, so where does that leave me? I guess I would have to buy a 17-40 & miss out on the IS, or go for a 17-55IS but lose build quality & I dont like all the dust problems with that lens.
As far as noise levels go, what we are seeing is comparisons not of what the camera sensor is capable of but much more of what the in camera processing is capable of! the Digic III chip obviously has a better NR algorithm which helps, but no NR is ever that good, im sure its at the expense of detail.
The main thing which is a little daft, is all these comparisons, they are both cameras aimed at completely different markets, so what if the 40D can get close to a 5D for noise levels, its still a 40D, its like saying a 5D can produce as good IQ as a 1D series, but its NOT a 1D, those that own 1 series would say "so what?" they purchased the 1 series body for a reason.
While we are talking about noise, a recent 400D vs 40D revealed that noise levels are roughly the same, so how come nobody has said in the past a 400D is equal to a 5D?
If you want a jack of all trades because you do a little of everything, I would say a 40D would be a good choice, but if your main focus is weddings, landscapes then the 5D remains the better camera, its just nicer to use everyday.
I like to see these tests, but I think if both cameras were the same price most people would choose a 5D, unless they particually needed the 1.6x crop, this is why we are seeing these comparisons, its out of the range of most people so they like to think their new 40D is just as good, if not better, but I urge anyone to try both & see which you prefer using on a daily basis.