I've compared the D3 and the M8 on a Leica-presentation. I've compared some pictures I made on the Mac - the M8 IQ was simply superior. It was sharper, had better colors, less artifacts, less noise... It's a completly dfferent class, even if 8MP vs. 10MP sound like being comparable.
Of course ergonomics and build quality was also superior - as you would expect at this price - but the D3 isn't exactly cheap either...
THe D3 is a simple Panasonic mass-product and has not much to do with Leica.
When you can afford it and you like rangefinders, the M8 will propably give you the best IQ avaible in 35mm-class. And the lenses are even fitted for the future, 35Asph, 50Asph, the Apos, the new Tri (I've watched some RAWs 16mm f4 - nearly perfect), they all have unique optical qualities, ready for even more Megapixels...