Pentax have missed a trick here.
Why go down the ill fated integrated approach like the Mamiya ZD, which most people suggest was a bit of a failure (maybe more to do with sensor implementation rather than concept).
I would have liked an open-back system, especially if the ubiquitous lenses are as good as people are saying and as affordable too - even less than the mamiya offerings (like for like), which are no slouch optically either. Does anybody know if the af system in the pentax 645 is based upon motors in the camera itself, like the mamiya AFD system, or if they are in the lenses like most 35 mm cameras. I only ask, as if you look at mamiya, the only way you can get better Af performance is to go get one of the ever newer incarnations of the AFD, such as the AFDIII or the latest Phamiya one DCF etc. If this is the case with pentax, then more's the better as the presumably can put the very latest autofocus technology into the body and make even quite old lenses focus quickly/accurately. If however the motors are based in the old legacy lenses - the only ones availble now, then there is a problem. Good though the lenses may be optically, no amount of jiggery pokey at the body end would be able to improve things?!
I suppose given there are no third party backs with a pentax mount - for obvious reasons - there would be no market for used third party compatible backs (a growing market and a "cheap" way into MFDB's), and I suppose Phase one isn't going to rush out to make one (a mount for pentax for its backs) or have capture one compatibility, if people can go out and buy a pentax 645 body and full set of lenses for half the price of the Phase One alternatives!!
However by the same token Phase/sinar/leaf might also nab some massive latent Pentax 645 film user base that want to use their backs with their existing lens set up! I have no idea how big this fan base might be and as to whether it "would pay". Certainly if you are a pentax user $7K will buy you a nice second hand back from a third party, such as a 22Mp back or even a 33/39MP back.
It's an interesting business model becuase an open platform option now, would mean that all new Leaf/sinar and maybe Phase backs might support the platform in the future (or even intro into the existing line of backs hte pentax mount) and there-by increase the market for the lesnes and any new digital lenses in the pipeline. My guess is that the margins for an integrated digital platform (in fact what they seem to be releasing) would be higher than the increased lens market base for third party back buyers ?
One good thing is that these integrated back cameras are generally more compact and lighter than modular designs. I, like others have commented would prefer to buy a 22Mp "full frame (well OK 1.1X crop)" camera rather than a 31MP cropped sensor product, especially if I was into landscapes (and I am). I am fed up of not using all of a lenses image circle (I shoot with the heavilly crop 16MP sensor MFDB). I can tell you, that if you are humping many KG's of Japaneese glass around the mountains, you are going to want to feel that all that glass is doing something usefull (and not just "the middle bit")!!
just my thoughts