Scott, I'm not so sure that's true. There still remains to be a single true medium format digital sensor -- all of them are smaller than 60x45, which is the smallest format size considered MF. The Leaf Aptus 22 MP is 48x32mm, the Hasselblad H3D is 39 MP /48x36, the Phase One is 49x37 / 39 MP, and the Mamiya ZD is 48x36 / 22 MP.
So all of these are just over half the size of the smallest MF film format. The 645 format is 3 times the size of 35mm, and these digital MF backs and bodies are only twice the size of 35mm (~1800 mm^2 vs 864 mm^2). This is almost identical to the difference between 35mm and APS-C (864 mm^2 vs 360 mm^2).
Taking the square root of this area difference, you'll find a roughly 1.5x crop factor when going from full frame 35mm to the current generation of digital MF bodies. If we're able to debate the merits of 35mm vs APS-C, this distinction will be considerably less important when debating 35mm vs 48x36.
In other words, we're still a ways off from getting our first digital MF sensor that actually is medium format in size. If you look at the extra cost for these cameras, all well over $10,000 and some considerably higher than that, it seems like a true digital 645 (let alone 6x7) camera is far away. Furthermore, all the conveniences of small format (like higher framerates, high quality zoom lenses, and a lot of competition to drive down prices and drive up quality), you're still going to be looking at a huge competitive advantage for high end 35mm DSLRs.
So I think mainstream FF is a lot closer to us than digital MF, which in all honsesty doesn't really exist yet. These MF cameras are some sort of intermediate for people who want to continue using their great CZ-Hassy and Mamiya lenses.