As if it is not confused enough. Let me try this. I just have a nice talk with a friend, non-profssional, but very much into photography, as many here have seen a lot. Today's many serious amatuer may be better than some pros, certainly not nearly as good as those top ones, but bette than some. Same thing about the cameras, and sendors. But this is not to discussed the size and pixel number.
Back on my talk with my friend, we discussed a lot of cameras, from film to digital, from small to big, from a Canon iXus 400 to P65+, from fast like the coming 1Ds4 (or already landed) to Sigma DP-1/DP-2, and from range finder to no finder.
Quality aside, I think one of the biggest difference between medium format and DSLR is speed. But depend on what to shoot, slow and speed is both good. I remember the days when Konica Hexar RF came out, I got one to shoot along with my M6, but quickly find out my M6 produce better image - or I shoot better with my M6 - I guess because I need to work on exposure so the process of making picture involved more elements than simply aim-and-shoot + focus on Hexar RF.
Now with this size and pixel debate, I think what is mattered is how one work on the camera. I joked to my friend that I shoot a higher averaged quality picture from my Sigma DP-1 because it is slow so I want to make sure I got a good one rather than few seconds later I need to repeat the process again - and in those seconds waiting, one can rapid fire tens of pictures, at higher megapixels, with those high end Nikon and Canon. But I realized it is not a joke. Speed gets you a lot of picture, but not necessary the picture wanted. Slow allows (and sometimes force) one to think, and perhpas enjoy more on the process of photography itself as a fine craft. No offence to those who has to shoot fast.
And the speed and preparation is what I think set the medium format and DSLR apart. For me, medium format is not just about larger pictures, it is the process that photographer is more involved in getting the picture although more involve does not equal to better work - sometimes it is and sometimes it is not. But work is one thing, fun is another, to me, using medium format whole day in studio, and at the end of day, I want a cup of coffee to grasp the last bit of photographic excitment. But with DSLR, at the end of day, I am ready to leave.