Pardon my ignorance... but what constitutes "medium format" in the digital age? I know that one of Canon's 35mm 1d series gives a resolution of 16 megapixels. Hasselblad has medium format digital backs that do 22 megapixels (probably even more now).
So, once Canon (or Nikon) have cameras that are 22 megapixels or higher - isn't that comparable to having a medium format camera? Or are there other differences that I'm just not familiar with? (I've never had the opportunity to use medium/large format cameras)
First of all, you're falling victim to the numbers game. All megapixels aren't created equal. For instance, a D2hs is "only" a 4MP camera, but those 4MP are really good ones. I'm going to break a personal rule here and reference Ken Rockwell, for all the hyperbole and idiocy he does sprout, there's the occasional gem on his site, in this case:
http://www.kenrockwell.com/tech/mpmyth.htm
For a general overview of the issues with sensor sizes, including yield-per-wafer, as well as diffraction, see:
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
(though there are better illustrations out there with imperfections factored a little better, it illustrates the cost difference, which is the underlying premise to my statement that APS-C will outdo 35MM- a 400% change in gross margins for a sensor manufacturer is a lot more compelling than noise stats.)
Finally, and really you should look at all these pages and maybe do some more research:
http://www.luminous-landscape.com/essays/sensor-design.shtml
Covers the whole thing.
I seem to recall reading somewhere that the sensor was the most expensive part of the camera, now do the math, knowing that the 645 chip is going to be ~2.6x the size of the 35mm chip- while prices are going down, they're not going down as quickly as say microprocessor prices because uP's get smaller, so you get higher yields per chip as you go onwards- the same will happen with sensors, smaller sensors mean higher yields, mean more out of less perfect starting wafers... It's simple economics that will kill "full frame."
22MP out of a LF back is going to be better quality than 22MP out of a MF back is going to be better than 22MP out of a 35mm body all other things being equal- though pure resolution isn't- that's what we're facing today, 12MP on APS-C is about where physics rears its ugly head (hey, my $4000 D2x does worse at f/22 than a $400 D40!)
While some of the original 22MP MF backs were 35mm sized sensors, the MF folks were screaming for "full frame" too- only their FF was at least 6cm x 4.5cm.
Read the links, then look at some 12MP images from P&S cameras, then from DSLRs- equal numbers don't mean equal quality.