A couple things to notice:
* DFA645 have silver band rather than DA* gold band near focussing ring (yes purely cosmetics I know).
* 645D has Mirror lock up (button near the prism.
* 645 is clearly sealed (based on earlier pictures seen).
* The 645 format is in reality 56 × 41.5 mm which makes a diagonal of 6,97cm (
http://en.wikipedia.org/wiki/120_film#Frame_sizes
Looking at Kodak sensors, the diagonal is between 5.52 and 6.13 cm (
http://www.kodak.com/global/en/business/ISS/Products/Fullframe/index.jhtml?pq-path=14425
Standard lens for film 645 is 75mm. If the standard now becomes 55mm, the crop is 1.3x or 5,36cm. Corresponding available sensors is 31.6MP. Furthermore, Pentax said sensor of more than 30MP, if they knew they'd go for 50MP they'd say so.
If they decide to go for higher MP sensor from Kodak, the diagonal would be 6,13 cm and normal lens would be 65mm.
All this only if the crop maintain the same aspect as the orgininal 645 format (which I doubt).
Edit: Oh and the AF/MF switch on the DFA55 hints at SDM
EDit 2: no aperture ring on the DFA645 55mm.