I think the DR of the current Canon 50MP is fine, although not class leading. Actually for my shooting style I prefer 120MP over class-leading DR. My current 50MP CCD is not even class-leading among the CCDs, but what I want more of is foremost megapixels. Remains to be seen when this new Canon appears though, it can be more than a year away.
I would not worry about Sony not having time to make MF sensors, those are just trivial(?) scaleups of their 24x36mm sensor tech anyway.
It's nice to see Canon moving, at last. I'm also excited by their new lens tech (blue spectrum refractive lens element) which allows sharper lenses. All 135 lens manufacturers are moving forward to meet the new high res sensor demands. It shall be interesting to see how far they can reach, especially at the difficult wide end.
And yes this release will probably affect second hand MFD pricing, it was quite clear that the lower end backs dropped a bit when the 50MP Canon was announced. The second hand market seems to be very megapixel-driven, the MFD backs that no longer outresolves the smaller format loses their sexiness (noone seems to care about the lenses), so perhaps we'll see a significant drop of IQ180 and such backs this time around.
Here in Sweden there was a major bump in Hasselblad second hand sales up to 50MP during the spring, very good prices too, but now it has settled, not much out in the market right now. I bought my H4D-50 a bit too early and sold my Aptus a bit too late to make the best use of the situation, but I managed quite okay anyway.
I'm sure we'll see 645 full-frame CMOS "soon", and it will surely be up there at 120MP.
Personally I'm waiting for the next generation MFD CMOS, which I hope will be able to do tech wide angle much better than the current, and hopefully have quite a bit more megapixels. If they can't, my current MFD system is probably the last. I have no problems holding out a few more years though.