Someone's been reading DP Review.
For some reason, I can't see Canon breaking into the medium format market. Pentax has tried, but, I don't know how well the 645D is going for them.
Mamiya/Leaf-Aptus, Sinar, Phase One, and Hasselblad already own the medium format market.
Let Canon/Nikon concentrate on the 35mm market. It is what they do well, after all.
If anyone, I'd expect Sony to get their fingers into the medium format pie.
H