Limsgp said:A guess. For movies, the sensor has to do inline frame transfer, maybe reading 1 row of image data at a time, but this requires additional electonics and use up some space on the sensor which could be used to gather light. Since DSLR is meant to produce the best possible image quality, it does away with the "unnecessary" electronics and cannot do inline frame transfer. Instead it does "full frame transfer" where all image data is read "at once". Maybe a DSLR that can take movies is possible if it has a frame rate of 30 frames/sec.
As for the mirror blocking.. probably the mirror (and the optical viewfinder) would be unnecessary if the sensor can take movies (meaning "live view" is possible) and AF can use image data from the image sensor itself instead if the mirror is not there to block it?
Just a guess.
Then how those dig-cam do the video? Those video cam ccd/cmos also can do both... the does full-frame-transfer when a still pic is required.