Thanks for the info ! So basically you prefer the Pentax 645D compared to the Phase One system ? (when it comes to 40MP and base specs that is) The Pentax is really tempting but it worries me that there are only 2 MF lenses available. The rest is something i'd rather not use or buy as it was never designed for the Pentax 645D. I can work with those 2 until there are more available so it's not a huge problem.
Why don't you want to use older 645 lenses? I have two and they perform beautifully. There are also Pentax 645 zooms that have a good reputation. It is true some of the old 645 lenses are not great--the 45mm has been panned by most--but there are lenses that are universally liked or even raved about. Getdpi forums has a 645D image thread and many other 645D threads. You can also go to Pentax Forums that have a lot of threads on the 645D.
Here is a Japanese site where you can view full-size images from older Pentax lenses on a 645D (click on the images to see them bigger, loading can be slow):
http://ganref.jp/magazines/index/1/0/369with some zooms:
http://ganref.jp/magazines/index/1/0/368Chis Willson (travel 67) has some full-size images from the 645D on his Flickr account, some from the 55mm and some from the Pentax 6x7 105mm.
As for new lenses, apparently Pentax has third lens slated for next year. They have suggested that there will be a new lens every year, but predicting is hard to do, especially about the future.
I have used Phase backs on view cameras. I really don't like the phase interface and it is more work getting rid of things like lens casts--something I do not need to do with the 645D. The Phase backs may be different on a Mamiya body, but from what I understand, the body and back are still separate with separate controls. I was just started shopping for a MFD when I saw the 645D. I was intending to go with Phase, but after seeing results with the Pentax, I changed my mind. The Phase system seems like a lot of different pieces put together unlike an integrated system. I liked the Pentax ISO and bulb specs better as well.
BTW, I have not used Hasselblad MFD and so I don't know if they have a more integrated approach.
Now all of these cameras do what they as supposed to do, but for some reason, the Pentax is treated like a second class citizen. Perhaps if Pentax charged more...