I've been following the 645Z announcements with great interest. I'm not in a position to need one, but nonetheless, I'm still interested. I love the 645NII system for shooting film, as I've found that it's a great system, and I love the look/draw of the lenses.
P1 and Leaf/Sinar,etc. still have an advantage however, in the "niche" MF technical camera arena, with the likes of Alpa, Cambo, Horseman, Silvestri, etc. For users of those systems and lenses, a removable back is a necessity. However, I do see this 645Z becoming a very hot ticket as soon as it hits stores. As long as the tethering works well, is stable, and the files can be manipulated by capable/comparable software to P1/Leaf Capture, I'd say it's definitely "game on".
Interesting times ahead, but a camera is just that: a camera. It still takes skilled hands to make great photographs. Not just technical specs and a low price point.
just my 2¢
-Dan