The Japanese DC Watch has reported today that Pentax has updated at their Japanese website for more concrete information about the upcoming 645D (just a little bit):-
http://dc.watch.impress.co.jp/docs/news/20100302_352104.html
These include the 645D sensor will be about 1.7 times larger than a 135 full frame one (in area) and that it has dual SDHC slots (still not SDXC compliance? :-() that can be written either simultaneously or sequentially.
Btw, I believe more news about the 645D will come, as the Japanese Camera & Photo Imaging Show 2010 is approaching, very soon.
Update News (3-9):
Updates are continuously posted at the Pentax Japanese homepage for more 645D information (but yet still very little):-
http://www.camera-pentax.jp/new/new.html
Now, it is told that the sensor is of 4,000 Mega Pixels. (See "Preview Vol. 4" - Click on the respective link 〈ニュース情報はこちら〉in the above page will bring you there.)
Last News: Official 645D Teaser is There!