Thanks for the comments, guys. Regarding the issue of backs and cameras, they are calibrated to one another for the sake of precise focus. Both the camera and back store some sort of profile information when they are paired. It sounds like certain U.S. rental places may eventually be able to pair cameras and bodies, but – as I understand it – for now the pairing has to happen at the factory. I know it's a contentious point for the Hasselblad system, but the payoffs, in terms of control and a shared battery, etc, offset the inconvenience in my mind. Coming from a Nikon system, the closed/integrated approach is nothing new and offers some distinct advantages in easy and consistent workflow.
I believe it's fair to consider the H3DII and H4D unified cameras – like a Leica S2, Pentax 645D, or Nikon D3X – that happen to offer a detachable back for view camera use or easy sensor cleaning.
I have a little rental experience with Phase One/Mamiya cameras and am looking forward to an official demo in the coming weeks to really compare the two systems in my shooting environment. Does anyone with a Phase or Mamiya kit take advantage of the interchangeability of backs and cameras?