Tuarreg -- I have not really had the D3 for that long, so I cannot give you a complete comparison, but I would say that at base ISO, the image quality is relative to the lens you are using, rather than the camera. The difference between 10mp 1.3 crop and 12mp full frame is not really significant in terms of resolution, but it does make a big difference in noise. I find that at ISO 160 (the M8's base) and 320, the M8 will generally take equal or better images to the D3. at 640 and above, the D3 is way ahead. The D3 at 3200 is probably close to the M8 at 640.
All that said, in terms of use in normal lighting conditions, it is the lenses that determine the image quality, and 90% of the Leica lenses are better than the Nikon lenses. This is what you are paying for in the M8 -- the ability to use the Leica lenses. If you could bolt Leica lenses on the D3 (even the Leica R lenses), then it would be a different story.
But for the most part, Leica primes are way ahead of the Nikon lenses. The exceptions would be the 14-24 and the 24-70, but even here, the Leica lenses are ahead -- particularly in speed, performance wide open, lack of distortion and size.
For me, the D3 is about getting the job done. It does this unbelievably well. Point it at something, press a button, and you will get a good photo. It is not the sort of thing I carry around with me unless I have a specific reason to...the M8 and a single lens is a lightweight, small kit that can slip in a small bag or even a trench coat pocket if you have a small enough lens. The D3 (or the 1DsMkIII) is a tool for getting work done. It is brilliant at it. I am not saying it can't do "fine art" or street photography or whatever else -- it is just not something you can have with you all the time without a particular reason -- unless you are a masochist. It's like walking around with a KGSS strapped to your back and Omega II's in order to listen to your iPod. The M8 is more like RS-1's unamped into it. They are different -- both are excellent, but in different ways.
