Peter
I think that if you are talking about "formal" architectural photography, then none of the MF cameras are particularly suitable - Pentax, Mamiya, Hasselblad, Contax, whatever. Hassy did have the Flex and Arcbody solutions, but they were expensive and not terribly easy to use. The Hasselblad PC Mutar inflicted a 1.4x crop factor on you, meaning that your wideangles were no longer wide. All these things are just not a patch on a decent view camera with movements and some Schneider lenses (or Rodenstock, or whatever you prefer). I can't see the new Pentax being any better in this regard.
However, if you mean by "architectural" just some nice pictorial shots of buildings and townscapes, then any MF camera processed through the perspective correction in PS or LR3 will stand up to scrutiny. MF digital files seem to have enough "stretch" in them that it is very hard to see any quality loss after processing, IMHO. Obviously you can apply "shift" but not "tilt" (as you can't alter DOF which is already baked-in) but you can emulate "swing" as well. The traditionalists will hold up their hands in horror, but in my view you are better off doing things this way rather than trying to use shift on a lens with too small an image circle and losing quality at the edge of the frame as a result.
John