Frankly, how could it not be the most popular MF camera in terms of growth?
The Japanese market alone is able to absorb tens of thousands units based on the existing Pentax 645 film user base. This is a totally untouched market because to the insane pricing of companies like Phaseone and Leaf in Japan having resulted in close to zero penetration among the most important higher end amateur market. Most of the Pentax MF film owners around here are above 50 years old and 8000 US$ is literally peanuts for them once they are confident that the camera is the right one in terms of usability, perceived material quality and image quality. The 645D is obviously meeting all these criterias and has zero drawback (not even the lack of live view for those guys who mostly don't know what live view is).
Anyway you look at it, MF is saturated in the US and Europe because the few folks interested and able to purchase a Phaseone/Leaf at those stratospheric prices already have cameras. Upgrading existing users to the latest offering is the main income niche for Phasone/Hassy. Lowering their prices a bit did for sure expand that niche somewhat, but there are still very expensive once you think in terms of system ROI.
My view is that the only real possible large growth for MF is the replacement of the 1ds3s thanks to the decision of Canon not to try to compete with the D3x and that is exactly where Pentax has an offering that is very hard to beat.
My guess is that many 5DII users are also on the edge, waiting to see whether Canon will be able to release a 5DIII more focused on DR and physical robustness. If they disappoint then Pentax will have a real nice card to play and S/N are likely to go up in the high tens of thousands.